stdbool.h: Update true and false expansions for C2x

stdbool.h: Update true and false expansions for C2x

C2x has changed the expansions of the true and false macros in
<stdbool.h> so that they have type _Bool (including in #if conditions,
i.e. an unsigned type in that context).  Use the new expansions in
GCC's <stdbool.h> for C2x.

See bug 82272 for related discussion (but this patch does *not*
implement the warning discussed there).

Note that it's possible there may be a further change to make bool,
true and false keywords (there was support in principle for that at
the April WG14 meeting).  But currently these expansions of type _Bool
are what C2x requires and there isn't actually a paper before WG14 at
present that would introduce the new keywords.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/
2020-10-29  Joseph Myers  <joseph@codesourcery.com>

	* ginclude/stdbool.h [__STDC_VERSION__ > 201710L] (true, false):
	Define with type _Bool.

gcc/testsuite/
2020-10-29  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/c11-bool-1.c, gcc.dg/c2x-bool-1.c, gcc.dg/c99-bool-4.c:
	New tests.