c: Turn -Wreturn-mismatch into a permerror

gcc/

	* doc/invoke.texi (Warning Options): Document that
	-Wreturn-mismatch is a permerror in C99 and later.

gcc/c/

	PR c/96284
	* c-typeck.cc (c_finish_return): Use permerrors
	for OPT_Wreturn_mismatch diagnostics.

gcc/testsuite/

	* gcc.dg/permerror-default.c (return_mismatch_1)
	(return_mismatch_2): Expect new permerror.
	* gcc.dg/permerror-gnu89-nopermissive.c (return_mismatch_1):
	Likewise.
	* gcc.dg/permerror-system.c: Likewise.
	* gcc.dg/20030906-1.c: Compile with -fpermissive due to
	expected -Wreturn-mismatch error.
	* gcc.dg/20030906-1a.c: New test.  Copied from
	gcc.dg/20030906-1.c.  Expect the error.
	* gcc.dg/20030906-2.c: Compile with -fpermissive due to
	expected -Wreturn-mismatch error.
	* gcc.dg/20030906-2a.c: New test.  Copied from
	gcc.dg/20030906-2.c.  Expect the error.
	* gcc.dg/Wreturn-mismatch-1.c: Compile with -fpermissive due to
	expected -Wreturn-mismatch error.
	* gcc.dg/Wreturn-mismatch-1a.c: New test.  Copied from
	gcc.dg/Wreturn-mismatch-1.c.  Expect the error.
	* gcc.dg/Wreturn-mismatch-2.c: Compile with -fpermissive due to
	expected -Wreturn-mismatch error.
	* gcc.dg/Wreturn-mismatch-2a.c: New test.  Copied from
	gcc.dg/Wreturn-mismatch-2.c.  Expect the error.
	* gcc.dg/diagnostic-range-bad-return.c: Compile with
	-fpermissive due to expected -Wreturn-mismatch error.
	* gcc.dg/diagnostic-range-bad-return-2.c: New test.
	Copied from gcc.dg/diagnostic-range-bad-return.c.  Expect the
	error.
	* gcc.dg/pr105635-2.c: Expect -Wreturn-mismatch error.
	* gcc.dg/pr23075.c: Build with -fpermissive due to
	expected -Wreturn-mismatch error.
	* gcc.dg/pr23075-2.c: New test.  Copied from gcc.dg/pr23075.c.
	Expect the error.
	* gcc.dg/pr29521.c: Compile with -fpermissive due to expected
	-Wreturn-mismatch error.
	* gcc.dg/pr29521-a.c: New test. Copied from gcc.dg/pr29521.c.
	Expect error.
	* gcc.dg/pr67730.c: Compile with -fpermissive due to expected
	-Wreturn-mismatch error.
	* gcc.dg/pr67730-a.c: New test.  Copied from
	gcc.dg/pr67730-a.c.  Expect error.
	* gcc.target/powerpc/conditional-return.c: Compile with
	-fpermissive due to expected -Wreturn-mismatch error.