gdb: add gdb::make_unique function

While GDB is still C++11, lets add a gdb::make_unique template
function that can be used to create std::unique_ptr objects, just like
the C++14 std::make_unique.

If GDB is being compiled with a C++14 compiler then the new
gdb::make_unique function will delegate to the std::make_unique.  I
checked with gcc, and at -O1 and above gdb::make_unique will be
optimised away completely in this case.

If C++14 (or later) becomes our minimum, then it will be easy enough
to go through the code and replace gdb::make_unique with
std::make_unique later on.

I've make use of this function in all the places I think this can
easily be used, though I'm sure I've probably missed some.

Should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>

gdbsupport/gdb_unique_ptr.h

@@ -56,6 +56,19 @@ struct noop_deleter
   void operator() (T *ptr) const { }
 };
 
+/* Create simple std::unique_ptr<T> objects.  */
+
+template<typename T, typename... Arg>
+std::unique_ptr<T>
+make_unique (Arg &&...args)
+{
+#if __cplusplus >= 201402L
+  return std::make_unique<T> (std::forward<Arg> (args)...);
+#else
+  return std::unique_ptr<T> (new T (std::forward<Arg> (args)...));
+#endif /* __cplusplus < 201402L */
+}
+
 } /* namespace gdb */
 
 /* Dup STR and return a unique_xmalloc_ptr for the result.  */