c: C2x storage class specifiers in compound literals

c: C2x storage class specifiers in compound literals

Implement the C2x feature of storage class specifiers in compound
literals.  Such storage class specifiers (static, register or
thread_local; also constexpr, but we don't yet have C2x constexpr
support implemented) can be used before the type name (not mixed with
type specifiers, unlike in declarations) and have the same semantics
and constraints as for declarations of named objects.  Also allow GNU
__thread to be used, given that thread_local can be.

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

gcc/c/
	* c-decl.cc (build_compound_literal): Add parameter scspecs.
	Handle storage class specifiers.
	* c-parser.cc (c_token_starts_compound_literal)
	(c_parser_compound_literal_scspecs): New.
	(c_parser_postfix_expression_after_paren_type): Add parameter
	scspecs.  Call pedwarn_c11 for use of storage class specifiers.
	Update call to build_compound_literal.
	(c_parser_cast_expression, c_parser_sizeof_expression)
	(c_parser_alignof_expression): Handle storage class specifiers for
	compound literals.  Update calls to
	c_parser_postfix_expression_after_paren_type.
	(c_parser_postfix_expression): Update syntax comment.
	* c-tree.h (build_compound_literal): Update prototype.
	* c-typeck.cc (c_mark_addressable): Diagnose taking address of
	register compound literal.

gcc/testsuite/
	* gcc.dg/c11-complit-1.c, gcc.dg/c11-complit-2.c,
	gcc.dg/c11-complit-3.c, gcc.dg/c2x-complit-2.c,
	gcc.dg/c2x-complit-3.c, gcc.dg/c2x-complit-4.c,
	gcc.dg/c2x-complit-5.c, gcc.dg/c2x-complit-6.c,
	gcc.dg/c2x-complit-7.c, gcc.dg/c90-complit-2.c,
	gcc.dg/gnu2x-complit-1.c, gcc.dg/gnu2x-complit-2.c: New tests.