c: Add -Wreturn-mismatch warning, split from -Wreturn-type

c: Add -Wreturn-mismatch warning, split from -Wreturn-type

The existing -Wreturn-type option covers both constraint violations
(which are mandatory to diagnose) and warnings that have known
false positives.  The new -Wreturn-mismatch warning is only about
the constraint violations (missing or extra return expressions),
and should eventually be turned into a permerror.

The -std=gnu89 test cases show that by default, we do not warn for
return; in a function not returning void.  This matches previous
practice for -Wreturn-type.

gcc/c-family/

	* c.opt (Wreturn-mismatch): New.

gcc/c/

	* c-typeck.cc (c_finish_return): Use pedwarn with
	OPT_Wreturn_mismatch for missing/extra return expressions.

gcc/

	* doc/invoke.texi (Warning Options): Document
	-Wreturn-mismatch.  Update -Wreturn-type documentation.

gcc/testsuite/

	* gcc.dg/Wreturn-mismatch-1.c: New.
	* gcc.dg/Wreturn-mismatch-2.c: New.
	* gcc.dg/Wreturn-mismatch-3.c: New.
	* gcc.dg/Wreturn-mismatch-4.c: New.
	* gcc.dg/Wreturn-mismatch-5.c: New.
	* gcc.dg/Wreturn-mismatch-6.c: New.
	* gcc.dg/noncompile/pr55976-1.c: Change -Werror=return-type
	to -Werror=return-mismatch.
	* gcc.dg/noncompile/pr55976-2.c: Change -Wreturn-type
	to -Wreturn-mismatch.