C Parser: Implement mixing of labels and code.

Implement mixing of labels and code as adopted for C2X
and process some std-attributes on labels.

2020-11-06  Martin Uecker  <muecker@gwdg.de>

gcc/
	* doc/extend.texi: Document mixing labels and code.
	* doc/invoke.texi: Likewise.

gcc/c/
	* c-parser.c (c_parser_label): Implement mixing of labels and code.
	(c_parser_all_labels): Likewise.

gcc/testsuite/
	* c-c++-common/attr-fallthrough-2.c: Update compiler flags.
	* c-c++-common/Wimplicit-fallthrough-20.c: Adapt test.
	* gcc.dg/20031223-1.c: Update compiler flags and adapt test.
	* gcc.dg/c11-labels-1.c: New test.
	* gcc.dg/c11-labels-2.c: New test.
	* gcc.dg/c11-labels-3.c: New test.
	* gcc.dg/c2x-attr-syntax-3.c: Adapt test.
	* gcc.dg/c2x-labels-1.c: New test.
	* gcc.dg/c2x-labels-2.c: New test.
	* gcc.dg/c2x-labels-3.c: New test.
	* gcc.dg/decl-9.c: Update compiler flags and add error.
	* gcc.dg/gomp/barrier-2.c: Update compiler flags and add warning.
	* gcc.dg/gomp/declare-simd-5.c: Update compiler flags and adapt test.
	* gcc.dg/gomp/declare-variant-2.c: Update compiler flags and add error.
	* gcc.dg/label-compound-stmt-1.c: Update compiler flags.
	* gcc.dg/parse-decl-after-label.c: Update compiler flags.