small _BitInt tweaks

I think it is undesirable when being asked for signed_type_for
of unsigned _BitInt(1) (which is valid) to get signed _BitInt(1) (which is
invalid, the standard only allows signed _BitInt(2) and larger), so the
patch returns 1-bit signed INTEGER_TYPE for those cases.
Furthermore it asserts in build_bitint_type that nothing attempts to create
signed _BitInt(0), unsigned _BitInt(0) or signed _BitInt(1) types.

2023-09-18  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* tree.cc (build_bitint_type): Assert precision is not 0, or
	for signed types 1.
	(signed_or_unsigned_type_for): Return INTEGER_TYPE for signed variant
	of unsigned _BitInt(1).
gcc/c-family/
	* c-common.cc (c_common_signed_or_unsigned_type): Return INTEGER_TYPE
	for signed variant of unsigned _BitInt(1).