Replace really_free_pendings with a scoped_ class

Replace really_free_pendings with a scoped_ class

This introduces scoped_free_pendings, and changes users of
really_free_pendings to use it instead, removing some clenaups.

I tried to examine the affected code to ensure there aren't dangling
cleanups in the vicinity.

gdb/ChangeLog
2017-11-04  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (process_full_comp_unit, process_full_type_unit):
	Use scoped_free_pendings.
	* dbxread.c (dbx_symfile_read, dbx_psymtab_to_symtab_1): Use
	scoped_free_pendings.
	* xcoffread.c (xcoff_psymtab_to_symtab_1): Use scoped_free_pendings.
	(xcoff_initial_scan): Likewise.
	* buildsym.c (reset_symtab_globals): Update comment.
	(scoped_free_pendings): Rename from really_free_pendings.
	(prepare_for_building): Update comment.
	(buildsym_init): Likewise.
	* buildsym.h (class scoped_free_pendings): New class.
	(really_free_pendings): Don't declare.