c: Turn -Wimplicit-int into a permerror

Most of these new permerrors are currently not diagnosed in system
headers.

gcc/

	PR c/91093
	PR c/96284
	* doc/invoke.texi (Warning Options): Document changes.

gcc/c/

	* c-decl.cc (warn_defaults_to): Remove.
	(grok_declarator, start_function): Call permerror_opt
	instead of warn_defaults_to.
	(store_parm_decls_oldstyle): Call permerror_opt for
	OPT_Wimplicit_int.

gcc/testsuite/

	* gcc.dg/permerror-default.c (implicit_int_1, implicit_int_2)
	(implicit_int_3, implicit_int_4): Expect new permerror.
	* gcc.dg/permerror-system.c: Expect a single new permerror.
	* gcc.dg/Wimplicit-int-1.c: Compile with -fpermissive due to
	expected warning.
	* gcc.dg/Wimplicit-int-4.c: Likewise.
	* gcc.dg/Wimplicit-int-1a.c: New test.  Copied from
	gcc.dg/Wimplicit-int-1.c, but expect errors.
	* gcc.dg/Wimplicit-int-4a.c: New test.  Copied from
	gcc.dg/Wimplicit-int-4.c, but expect errors.
	* gcc.dg/gnu23-attr-syntax-2.c: Compile with -fpermissive
	due to expected implicit-int error.
	* gcc.dg/gnu23-attr-syntax-3.c: New test.  Copied from
	gcc.dg/gnu23-attr-syntax-2.c, but expect an error.
	* gcc.dg/pr105635.c: Build with -fpermissive due to implicit
	int.
	* gcc.dg/pr105635-2.c: New test.  Copied from
	gcc.dg/pr105635.c.  Expect implicit int error.
	* gcc.dg/noncompile/pr79758.c: Build with -fpermissive due to
	implicit int.
	* gcc.dg/noncompile/pr79758-2.c: New test.  Copied from
	gcc.dg/noncompile/pr79758.c.  Expect implicit int error.