Move pending_macros to buildsym_compunit

Move pending_macros to buildsym_compunit

This moves the pending_macros global into buildsym_compunit.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* buildsym.c (~buildsym_compunit): Free the macro table.
	(struct buildsym_compunit) <get_macro_table, release_macros>: New
	methods.
	<m_pending_macros>: New member.
	(pending_macros): Remove.
	(~scoped_free_pendings, get_macro_table, prepare_for_building)
	(reset_symtab_globals, end_symtab_get_static_block)
	(end_symtab_with_blockvector, augment_type_symtab)
	(buildsym_init): Update.