[binutils-gdb] Make find_separate_debug_file* return std::string

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT a8dbfd5853e3a5f7f2a3ca817e96d9e0759061a2 ***

Author: Simon Marchi <simon.marchi@polymtl.ca>
Branch: master
Commit: a8dbfd5853e3a5f7f2a3ca817e96d9e0759061a2

Make find_separate_debug_file* return std::string

This patch makes the find_separate_debug_file* functions return
std::string, which allows to get rid of some manual memory management
and one cleanup.

gdb/ChangeLog:

	* build-id.c (find_separate_debug_file_by_buildid): Return
	std::string.
	* build-id.h (find_separate_debug_file_by_buildid): Return
	std::string.
	* coffread.c (coff_symfile_read): Adjust to std::string.
	* elfread.c (elf_symfile_read): Adjust to std::string.
	* symfile.c (separate_debug_file_exists): Change parameter to
	std::string.
	(find_separate_debug_file): Return std::string.
	(find_separate_debug_file_by_debuglink): Return std::string.
	* symfile.h (find_separate_debug_file_by_debuglink): Return
	std::string.