libcpp: adjust _cpp_file accessors

libcpp: adjust _cpp_file accessors

Back in r78875 mrs added cpp_get_path/dir accessors for _cpp_file in order
to interface with the darwin framework system.  But now I notice that the
latter duplicates the better-named _cpp_get_file_dir, and I'm inclined to
rename the former to match.

Perhaps we should drop the initial underscore since these are no
longer internal interfaces; OTOH, _cpp_hashnode_value and
_cpp_backup_tokens still have the initial underscore in cpplib.h.

libcpp/ChangeLog:

	* include/cpplib.h (cpp_get_path, cpp_get_dir): Remove.
	(_cpp_get_file_path, _cpp_get_file_name, _cpp_get_file_stat)
	(_cpp_get_file_dir): Move prototypes from...
	* internal.h: ...here.
	* files.cc (_cpp_get_file_path): Rename from...
	(cpp_get_path): ...this.
	(cpp_get_dir): Remove.

gcc/ChangeLog:

	* config/darwin-c.cc (find_subframework_header): Use
	_cpp_get_file_*.