Change parameters to language_defn::post_parser

In the expression rewrite, Ada type resolution will be done at parse
time rather than in a post-parse pass.  At this point,
language_defn::post_parser will be removed.  However, for this to
work, the information available to post_parser must be made available
during the actual parse.

This patch refactors this code slightly to make this possible.  In
particular, "void_context_p" is passed to the parser_state
constructor, and the parser state is then passed to the post_parser
method.

gdb/ChangeLog
2020-12-16  Tom Tromey  <tom@tromey.com>

	* rust-exp.y (rust_lex_tests): Update.
	* parser-defs.h (parser_state): Add void_p parameter.
	<void_context_p>: New member.
	* parse.c (parse_exp_in_context): Update.
	* language.h (language_defn::post_parser): Remove void_context_p,
	completing, tracker parameters.  Add parser state.
	* ada-lang.c (ada_language::post_parser): Update.