c: Implement C2y N3356, if declarations [PR117019]

c: Implement C2y N3356, if declarations [PR117019]

This patch implements C2y N3356, if declarations as described at
<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3356.htm>.

This feature is cognate with C++17 Selection statements with initializer
<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r1.html>,
but they are not the same yet.  For example, C++17 allows

  if (lock (); int i = getval ())

whereas C2y does not.

The proposal adds new grammar productions.  selection-header is handled
in c_parser_selection_header which is the gist of the patch.
simple-declaration is handled by c_parser_declaration_or_fndef, which
gets a new parameter.

	PR c/117019

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_declaration_or_fndef): Adjust declaration.
	(c_parser_external_declaration): Adjust a call to
	c_parser_declaration_or_fndef.
	(c_parser_declaration_or_fndef): New bool parameter.  Return a tree
	instead of void.  Adjust for N3356.  Adjust a call to
	c_parser_declaration_or_fndef.
	(c_parser_compound_statement_nostart): Adjust calls to
	c_parser_declaration_or_fndef.
	(c_parser_selection_header): New.
	(c_parser_paren_selection_header): New.
	(c_parser_if_statement): Call c_parser_paren_selection_header
	instead of c_parser_paren_condition.
	(c_parser_switch_statement): Call c_parser_selection_header instead of
	c_parser_expression.
	(c_parser_for_statement): Adjust calls to c_parser_declaration_or_fndef.
	(c_parser_objc_methodprotolist): Likewise.
	(c_parser_oacc_routine): Likewise.
	(c_parser_omp_loop_nest): Likewise.
	(c_parser_omp_declare_simd): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.dg/c23-if-decls-1.c: New test.
	* gcc.dg/c23-if-decls-2.c: New test.
	* gcc.dg/c2y-if-decls-1.c: New test.
	* gcc.dg/c2y-if-decls-2.c: New test.
	* gcc.dg/c2y-if-decls-3.c: New test.
	* gcc.dg/c2y-if-decls-4.c: New test.
	* gcc.dg/c2y-if-decls-5.c: New test.
	* gcc.dg/c2y-if-decls-6.c: New test.
	* gcc.dg/c2y-if-decls-7.c: New test.
	* gcc.dg/c2y-if-decls-8.c: New test.
	* gcc.dg/c2y-if-decls-9.c: New test.
	* gcc.dg/c2y-if-decls-10.c: New test.
	* gcc.dg/c2y-if-decls-11.c: New test.
	* gcc.dg/gnu2y-if-decls-1.c: New test.
	* gcc.dg/gnu99-if-decls-1.c: New test.
	* gcc.dg/gnu99-if-decls-2.c: New test.