Introduce objfile::intern

This introduces a string cache on the per-BFD object, replacing the
macro and filename caches.  Both of these caches just store strings,
so this consolidation by itself saves a little memory (about the size
of a bcache per objfile).

Then this patch switches some allocations on the objfile obstack to
use this bcache instead.  This saves more space; and turns out to be a
bit faster as well.

Here are the before and after "maint time" + "maint space" results of
"file ./gdb":

    Command execution time: 4.664021 (cpu), 4.728518 (wall)
    Space used: 39190528 (+29212672 for this command)

    Command execution time: 4.216209 (cpu), 4.107023 (wall)
    Space used: 36667392 (+26689536 for this command)

The main interface to the string cache is a new pair of overloaded
methods, objfile::intern.

gdb/ChangeLog
2020-03-04  Tom Tromey  <tom@tromey.com>

	* symmisc.c (print_symbol_bcache_statistics)
	(print_objfile_statistics): Update.
	* symfile.c (allocate_symtab): Use intern.
	* psymtab.c (partial_symtab::partial_symtab): Use intern.
	* objfiles.h (struct objfile_per_bfd_storage) <filename_cache,
	macro_cache>: Remove.
	<string_cache>: New member.
	(struct objfile) <intern>: New methods.
	* elfread.c (elf_symtab_read): Use intern.
	* dwarf2/read.c (fixup_go_packaging): Intern package name.
	(dwarf2_compute_name, dwarf2_physname)
	(create_dwo_unit_in_dwp_v1, create_dwo_unit_in_dwp_v2): Intern
	names.
	(guess_partial_die_structure_name): Update.
	(partial_die_info::fixup): Intern name.
	(dwarf2_canonicalize_name): Change parameter to objfile.  Intern
	name.
	(dwarf2_name): Intern name.  Update.
	* buildsym.c (buildsym_compunit::get_macro_table): Use
	string_cache.

gdb/symfile.c

@@ -2783,9 +2783,7 @@ allocate_symtab (struct compunit_symtab *cust, const char *filename)
   struct symtab *symtab
     = OBSTACK_ZALLOC (&objfile->objfile_obstack, struct symtab);
 
-  symtab->filename
-    = ((const char *) objfile->per_bfd->filename_cache.insert
-       (filename, strlen (filename) + 1));
+  symtab->filename = objfile->intern (filename);
   symtab->fullname = NULL;
   symtab->language = deduce_language_from_filename (filename);