Introduce gdb::byte_vector, add allocator that default-initializes

In some cases we've been replacing heap-allocated gdb_byte buffers
managed with xmalloc/make_cleanup(xfree) with gdb::vector<gdb_byte>.
That usually pessimizes the code a little bit because std::vector
value-initializes elements (which for gdb_byte means
zero-initialization), while if you're creating a temporary buffer,
you're most certaintly going to fill it in with some data.  An
alternative is to use

  unique_ptr<gdb_byte[]> buf (new gdb_byte[size]);

but it looks like that's not very popular.

Recently, a use of obstacks in dwarf2read.c was replaced with
std::vector<gdb_byte> and that as well introduced a pessimization for
always memsetting the buffer when it's garanteed that the zeros will
be overwritten immediately.  (see dwarf2read.c change in this patch to
find it.)

So here's a different take at addressing this issue "by design":

#1 - Introduce default_init_allocator<T>

I.e., a custom allocator that does default construction using default
initialization, meaning, no more zero initialization.  That's the
default_init_allocation<T> class added in this patch.

See "Notes" at
<http://en.cppreference.com/w/cpp/container/vector/resize>.

#2 - Introduce def_vector<T>

I.e., a convenience typedef, because typing the allocator is annoying:

  using def_vector<T> = std::vector<T, gdb::default_init_allocator<T>>;

#3 - Introduce byte_vector

Because gdb_byte vectors will be the common thing, add a convenience
"byte_vector" typedef:

  using byte_vector = def_vector<gdb_byte>;

which is really the same as:

  std::vector<gdb_byte, gdb::default_init_allocator<gdb_byte>>;

The intent then is to make "gdb::byte_vector" be the go-to for dynamic
byte buffers.  So the less friction, the better.

#4 - Adjust current code to use it.

To set the example going forward.  Replace std::vector uses and also
unique_ptr<byte[]> uses.

One nice thing is that with this allocator, for changes like these:

  -std::unique_ptr<byte[]> buf (new gdb_byte[some_size]);
  +gdb::byte_vector buf (some_size);
   fill_with_data (buf.data (), buf.size ());

the generated code is the same as before.  I.e., the compiler
de-structures the vector and gets rid of the unused "reserved vs size"
related fields.

The other nice thing is that it's easier to write
  gdb::byte_vector buf (size);
than
  std::unique_ptr<gdb_byte[]> buf (new gdb_byte[size]);
or even (C++14):
  auto buf = std::make_unique<gdb_byte[]> (size); // zero-initializes...

#5 - Suggest s/std::vector<gdb_byte>/gdb::byte_vector/ going forward.

Note that this commit actually fixes a couple of bugs where the current
code is incorrectly using "std::vector::reserve(new_size)" and then
accessing the vector's internal buffer beyond the vector's size: see
dwarf2loc.c and charset.c.  That's undefined behavior and may trigger
debug mode assertion failures.  With default_init_allocator,
"resize()" behaves like "reserve()" performance wise, in that it
leaves new elements with unspecified values, but, it does that safely
without triggering undefined behavior when you access those values.

gdb/ChangeLog:
2017-06-14  Pedro Alves  <palves@redhat.com>

	* ada-lang.c: Include "common/byte-vector.h".
	(ada_value_primitive_packed_val): Use gdb::byte_vector.
	* charset.c (wchar_iterator::iterate): Resize the vector instead
	of reserving it.
	* common/byte-vector.h: Include "common/def-vector.h".
	(wchar_iterator::m_out): Now a gdb::def_vector<gdb_wchar_t>.
	* cli/cli-dump.c: Include "common/byte-vector.h".
	(dump_memory_to_file, restore_binary_file): Use gdb::byte_vector.
	* common/byte-vector.h: New file.
	* common/def-vector.h: New file.
	* common/default-init-alloc.h: New file.
	* dwarf2loc.c: Include "common/byte-vector.h".
	(rw_pieced_value): Use gdb::byte_vector, and resize the vector
	instead of reserving it.
	* dwarf2read.c: Include "common/byte-vector.h".
	(data_buf::m_vec): Now a gdb::byte_vector.
	* gdb_regex.c: Include "common/def-vector.h".
	(compiled_regex::compiled_regex): Use gdb::def_vector<char>.
	* mi/mi-main.c: Include "common/byte-vector.h".
	(mi_cmd_data_read_memory): Use gdb::byte_vector.
	* printcmd.c: Include "common/byte-vector.h".
	(print_scalar_formatted): Use gdb::byte_vector.
	* valprint.c: Include "common/byte-vector.h".
	(maybe_negate_by_bytes, print_decimal_chars): Use
	gdb::byte_vector.

gdb/ada-lang.c

@@ -61,6 +61,7 @@
 #include "arch-utils.h"
 #include "cli/cli-utils.h"
 #include "common/function-view.h"
+#include "common/byte-vector.h"
 
 /* Define whether or not the C operator '/' truncates towards zero for
    differently signed operands (truncation direction is undefined in C).
@@ -2567,8 +2568,7 @@ ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr,
   gdb_byte *unpacked;
   const int is_scalar = is_scalar_type (type);
   const int is_big_endian = gdbarch_bits_big_endian (get_type_arch (type));
-  std::unique_ptr<gdb_byte[]> staging;
-  int staging_len = 0;
+  gdb::byte_vector staging;
 
   type = ada_check_typedef (type);
 
@@ -2586,14 +2586,14 @@ ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr,
 	 packed, and therefore maybe not at a byte boundary.  So, what
 	 we do, is unpack the data into a byte-aligned buffer, and then
 	 use that buffer as our object's value for resolving the type.  */
-      staging_len = (bit_size + HOST_CHAR_BIT - 1) / HOST_CHAR_BIT;
-      staging.reset (new gdb_byte[staging_len]);
+      int staging_len = (bit_size + HOST_CHAR_BIT - 1) / HOST_CHAR_BIT;
+      staging.resize (staging_len);
 
       ada_unpack_from_contents (src, bit_offset, bit_size,
-			        staging.get (), staging_len,
+			        staging.data (), staging.size (),
 				is_big_endian, has_negatives (type),
 				is_scalar);
-      type = resolve_dynamic_type (type, staging.get (), 0);
+      type = resolve_dynamic_type (type, staging.data (), 0);
       if (TYPE_LENGTH (type) < (bit_size + HOST_CHAR_BIT - 1) / HOST_CHAR_BIT)
 	{
 	  /* This happens when the length of the object is dynamic,
@@ -2656,12 +2656,12 @@ ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr,
       return v;
     }
 
-  if (staging != NULL && staging_len == TYPE_LENGTH (type))
+  if (staging.size () == TYPE_LENGTH (type))
     {
       /* Small short-cut: If we've unpacked the data into a buffer
 	 of the same size as TYPE's length, then we can reuse that,
 	 instead of doing the unpacking again.  */
-      memcpy (unpacked, staging.get (), staging_len);
+      memcpy (unpacked, staging.data (), staging.size ());
     }
   else
     ada_unpack_from_contents (src, bit_offset, bit_size,