c: Implement C2x nodiscard attribute

c: Implement C2x nodiscard attribute

C2x adds the nodiscard standard attribute, with an optional string
argument, as in C++; implement it for C.

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

gcc/c/
2020-11-04  Joseph Myers  <joseph@codesourcery.com>

	* c-decl.c (handle_nodiscard_attribute): New.
	(std_attribute_table): Add nodiscard.
	* c-parser.c (c_parser_std_attribute): Expect argument to
	nodiscard attribute to be a string.  Do not special-case ignoring
	nodiscard.
	* c-typeck.c (maybe_warn_nodiscard): New.
	(build_compound_expr, emit_side_effect_warnings): Call
	maybe_warn_nodiscard.
	(c_process_expr_stmt, c_finish_stmt_expr): Also call
	emit_side_effect_warnings if warn_unused_result.

gcc/testsuite/
2020-11-04  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/c2x-attr-nodiscard-1.c, gcc.dg/c2x-attr-nodiscard-2.c,
	gcc.dg/c2x-attr-nodiscard-3.c, gcc.dg/c2x-attr-nodiscard-4.c: New
	tests.
	* gcc.dg/c2x-attr-syntax-5.c: Remove nodiscard test.