Constify parse_and_eval_type

Constify parse_and_eval_type

I noticed that the argumen to parse_and_eval_type could be "const".
This patch implements this change.

I wonder if this could be removed.  It's only called via
check_stub_method_group, which seems questionable to me.  However, I
didn't look into doing this.

gdb/ChangeLog
2020-12-13  Tom Tromey  <tom@tromey.com>

	* gdbtypes.c (safe_parse_type): Make argument const.
	* value.h (parse_and_eval_type): Make argument const.
	* eval.c (parse_and_eval_type): Make argument const.