C _BitInt support [PR102989]

This patch adds the C FE support, c-family support, small libcpp change
so that 123wb and 42uwb suffixes are handled plus glimits.h change
to define BITINT_MAXWIDTH macro.

The previous patches really do nothing without this, which enables
all the support.

2023-09-06  Jakub Jelinek  <jakub@redhat.com>

	PR c/102989
gcc/
	* glimits.h (BITINT_MAXWIDTH): Define if __BITINT_MAXWIDTH__ is
	predefined.
gcc/c-family/
	* c-common.cc (c_common_reswords): Add _BitInt as keyword.
	(unsafe_conversion_p): Handle BITINT_TYPE like INTEGER_TYPE.
	(c_common_signed_or_unsigned_type): Handle BITINT_TYPE.
	(c_common_truthvalue_conversion, c_common_get_alias_set,
	check_builtin_function_arguments): Handle BITINT_TYPE like
	INTEGER_TYPE.
	(sync_resolve_size): Add ORIG_FORMAT argument.  If
	FETCH && !ORIG_FORMAT, type is BITINT_TYPE, return -1 if size isn't
	one of 1, 2, 4, 8 or 16 or if it is 16 but TImode is not supported.
	(atomic_bitint_fetch_using_cas_loop): New function.
	(resolve_overloaded_builtin): Adjust sync_resolve_size caller.  If
	-1 is returned, use atomic_bitint_fetch_using_cas_loop to lower it.
	Formatting fix.
	(keyword_begins_type_specifier): Handle RID_BITINT.
	* c-common.h (enum rid): Add RID_BITINT enumerator.
	* c-cppbuiltin.cc (c_cpp_builtins): For C call
	targetm.c.bitint_type_info and predefine __BITINT_MAXWIDTH__
	and for -fbuilding-libgcc also __LIBGCC_BITINT_LIMB_WIDTH__ and
	__LIBGCC_BITINT_ORDER__ macros if _BitInt is supported.
	* c-lex.cc (interpret_integer): Handle CPP_N_BITINT.
	* c-pretty-print.cc (c_pretty_printer::simple_type_specifier,
	c_pretty_printer::direct_abstract_declarator,
	c_pretty_printer::direct_declarator, c_pretty_printer::declarator):
	Handle BITINT_TYPE.
	(pp_c_integer_constant): Handle printing of large precision wide_ints
	which would buffer overflow digit_buffer.
	* c-warn.cc (conversion_warning, warnings_for_convert_and_check,
	warnings_for_convert_and_check): Handle BITINT_TYPE like
	INTEGER_TYPE.
gcc/c/
	* c-convert.cc (c_convert): Handle BITINT_TYPE like INTEGER_TYPE.
	* c-decl.cc (check_bitfield_type_and_width): Allow BITINT_TYPE
	bit-fields.
	(finish_struct): Prefer to use BITINT_TYPE for BITINT_TYPE bit-fields
	if possible.
	(declspecs_add_type): Formatting fixes.  Handle cts_bitint.  Adjust
	for added union in *specs.  Handle RID_BITINT.
	(finish_declspecs): Handle cts_bitint.  Adjust for added union
	in *specs.
	* c-parser.cc (c_keyword_starts_typename, c_token_starts_declspecs,
	c_parser_declspecs, c_parser_gnu_attribute_any_word): Handle
	RID_BITINT.
	(c_parser_omp_clause_schedule): Handle BITINT_TYPE like INTEGER_TYPE.
	* c-tree.h (enum c_typespec_keyword): Mention _BitInt in comment.
	Add cts_bitint enumerator.
	(struct c_declspecs): Move int_n_idx and floatn_nx_idx into a union
	and add bitint_prec there as well.
	* c-typeck.cc (c_common_type, comptypes_internal):
	Handle BITINT_TYPE.
	(perform_integral_promotions): Promote BITINT_TYPE bit-fields to
	their declared type.
	(build_array_ref, build_unary_op, build_conditional_expr,
	build_c_cast, convert_for_assignment, digest_init, build_binary_op):
	Handle BITINT_TYPE.
	* c-fold.cc (c_fully_fold_internal): Handle BITINT_TYPE like
	INTEGER_TYPE.
	* c-aux-info.cc (gen_type): Handle BITINT_TYPE.
libcpp/
	* expr.cc (interpret_int_suffix): Handle wb and WB suffixes.
	* include/cpplib.h (CPP_N_BITINT): Define.