c: Implement C2y static assertions in expressionsHEADtrunkmaster

C2y has added support for static assertions as void expressions, in
addition to use as declarations (N3715 was accepted in an online vote
between meetings).

Implement the feature in GCC.  There is a syntactic ambiguity between
a static assertion as a declaration and one as an expression
statement, which the accepted feature resolves by making such a usage
a declaration (this only affects the sequence of syntax productions by
which the code is parsed, not the actual semantics of the assertion);
I've raised the similar ambiguity in for loops on the WG14 reflector.

If just concerned with C2y, and not with diagnosing the use of a
feature not supported in older standard versions, the feature might be
simpler to implement by defaulting to treating static assertions in
ambiguous contexts as expressions rather than declarations, but that
would make it hard to diagnose exactly the cases that are new in C2y
(those depend on the static assertion either not being the whole
expression statement, or being in a context where an expression
statement is allowed but a declaration is not, e.g. the body of an if
statement).  Instead, to support such diagnostics, the implementation
follows the standard in what is considered a declaration and what is
considered an expression, by looking ahead to what follows the closing
parenthesis when a static assertion starts in a context where a
declaration is permitted.

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

gcc/c/
	* c-parser.cc (c_parser_next_tokens_start_typename)
	(c_parser_next_tokens_start_declaration): Add argument for token
	to start from
	(c_parser_next_tokens_start_declaration): Check for whether static
	assertion followed by semicolon.
	(c_parser_check_balanced_raw_token_sequence): Declare earlier.
	(c_parser_compound_statement_nostart, c_parser_for_statement): Use
	c_parser_next_tokens_start_declaration not
	c_token_starts_declaration on second token.
	(c_parser_unary_expression): Handle static assertions.
	* c-parser.h (c_parser_next_tokens_start_declaration): Add
	argument.

gcc/testsuite/
	* gcc.dg/c23-static-assert-5.c, gcc.dg/c23-static-assert-6.c,
	gcc.dg/c23-static-assert-7.c, gcc.dg/c23-static-assert-8.c,
	gcc.dg/c2y-static-assert-2.c, gcc.dg/c2y-static-assert-3.c,
	gcc.dg/c2y-static-assert-4.c: New tests.

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