Change macro_source_fullname to return a std::string

Change macro_source_fullname to return a std::string

While working on the previous patch, I noticed that if
macro_source_fullname returned a std::string, then the callers would
be simplified.  This patch implements this idea.

gdb/ChangeLog
2019-02-17  Tom Tromey  <tom@tromey.com>

	* macrotab.h (macro_source_fullname): Return a std::string.
	* macrotab.c (macro_include, check_for_redefinition)
	(macro_undef, macro_lookup_definition, foreach_macro)
	(foreach_macro_in_scope): Update.
	(macro_source_fullname): Return a std::string.
	* macrocmd.c (show_pp_source_pos): Update.