Change void_context_p to bool

This patch changes void_context_p to bool, as a prerequisite to the
change to post_parser that I submitted here:

https://sourceware.org/pipermail/gdb-patches/2020-December/174080.html

Tested by rebuilding.

Note that nothing in-tree passes true here.  I don't know why this is,
but there is a use of this internally in AdaCore's tree.  I will try
to submit that patch, if it is needed.  (And if not, I will come back
around and remove this.)

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

	* parse.c (parse_exp_1, parse_expression_for_completion): Update.
	(parse_exp_in_context): Change void_context_p to bool.
	* language.h (struct language_defn) <post_parser>: Change
	void_context_p to bool.
	* ada-lang.c (class ada_language) <post_parser>: Update.