Add obstack_strdup overload taking a std::string

Add obstack_strdup overload taking a std::string

This adds an obstack_strdup overload that takes a std::string, and
changes a few spots in gdb to use it.

gdb/ChangeLog
2019-08-06  Tom Tromey  <tom@tromey.com>

	* symfile.c (reread_symbols): Use obstack_strdup.
	* stabsread.c (read_type): Use obstack_strdup.
	* gdb_obstack.h (obstack_strdup): New overload.
	* dwarf2read.c (dwarf2_compute_name, create_dwo_unit_in_dwp_v1)
	(create_dwo_unit_in_dwp_v2, build_error_marker_type)
	(dwarf2_canonicalize_name): Use obstack_strdup.
	* dbxread.c (read_dbx_symtab): Use obstack_strdup.
	* cp-support.c (inspect_type, replace_typedefs_qualified_name):
	Use obstack_strdup.