c: Turn -Wimplicit-function-declaration into a permerror

In the future, it may make sense to avoid cascading errors from
the implicit declaration, especially its assumed int return type.
This change here only changes the kind of the diagnostic, not
its wording or consequences.

gcc/

	* doc/invoke.texi (Warning Options): Document changes.

gcc/c/

	PR c/91092
	PR c/96284
	* c-decl.cc (implicit_decl_permerror): Rename from
	implicit_decl_warning.  Call permerror_opt instead of
	pedwarn and warning_at.
	(implicitly_declare): Adjust callers.

gcc/testsuite/

	* gcc.dg/permerror-default.c (implicit_function_declaration):
	Expect the new permerror.
	* gcc.dg/permerror-system.c: Likewise.
	* c-c++-common/spellcheck-reserved.c (test, test_2): Expect
	error instead of warning.
	(f): Expect error instead of warning.
	* gcc.dg/Wimplicit-function-declaration-c99.c: Compile with
	-fpermissive due to expected warning.
	* gcc.dg/Wimplicit-function-declaration-c99-2.c: New test.
	Copied from gcc.dg/Wimplicit-function-declaration-c99.c.
	Expect error.
	* gcc.dg/missing-header-fixit-1.c: Compile with -fpermissive
	due to expect error.
	* gcc.dg/missing-header-fixit-1a.c: New test.  Copied from
	gcc.dg/missing-header-fixit-1.c, but expect error.
	* gcc.dg/missing-header-fixit-2.c: Compile with -fpermissive
	due to expect error.
	* gcc.dg/missing-header-fixit-2a.c: New test.  Copied from
	gcc.dg/missing-header-fixit-2.c, but expect error.
	* gcc.dg/missing-header-fixit-4.c: Compile with -fpermissive
	due to expect error.
	* gcc.dg/missing-header-fixit-4a.c: New test.  Copied from
	gcc.dg/missing-header-fixit-4.c, but expect error.
	* gcc.dg/missing-header-fixit-5.c: Compile with -fpermissive
	due to expect error.
	* gcc.dg/missing-header-fixit-5a.c: New test.  Copied from
	gcc.dg/missing-header-fixit-5.c, but expect error.
	* gcc.dg/pr61852.c: Expect implicit-function-declaration
	error instead of warning.
	* gcc.dg/spellcheck-identifiers-2.c: Compile with
	-fpermissive due to expected warnings.
	* gcc.dg/spellcheck-identifiers-2a.c: New test.  Copied
	from gcc.dg/spellcheck-identifiers-2a.c.  Expect errors.
	* gcc.dg/spellcheck-identifiers-3.c: Compile with
	-fpermissive due to expected warnings.
	* gcc.dg/spellcheck-identifiers-3a.c: New test.  Copied
	from gcc.dg/spellcheck-identifiers-2a.c.  Expect errors.
	* gcc.dg/spellcheck-identifiers-4.c: Compile with
	-fpermissive due to expected warnings.
	* gcc.dg/spellcheck-identifiers-4a.c: New test.  Copied
	from gcc.dg/spellcheck-identifiers-2a.c.  Expect error.
	* gcc.dg/spellcheck-identifiers.c: Compile with
	-fpermissive due to expected warnings.
	* gcc.dg/spellcheck-identifiers-1a.c: New test.  Copied
	from gcc.dg/spellcheck-identifiers.c.  Expect errors.
	* gcc.target/aarch64/sve/acle/general-c/ld1sh_gather_1.c (f1):
	Expect error.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_index_1.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_index_restricted_1.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_1.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_2.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_3.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_4.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_5.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_restricted_1.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_restricted_2.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_restricted_3.c:
	(f1): Likewise.
	* gcc.target/aarch64/sve/acle/general-c/load_ext_gather_offset_restricted_4.c:
	(f1): Likewise.