Remove remaining cleanups from c-exp.y

Remove remaining cleanups from c-exp.y

This removes the remaining cleanups from c-exp.y by moving some
globals into c_parse_state, and changing expansion_obstack to be an
auto_obstack.

gdb/ChangeLog
2019-01-06  Tom Tromey  <tom@tromey.com>

	* c-exp.y (struct c_parse_state) <macro_original_text,
	expansion_obstack>: New member.
	(macro_original_text, expansion_obstack): Remove globals.
	(scan_macro_expansion, scanning_macro_expansion)
	(finished_macro_expansion): Update.
	(scan_macro_cleanup): Remove.
	(yylex, c_parse): Update.