c: C2x semantics for __builtin_tgmath

__builtin_tgmath implements <tgmath.h> semantics for integer generic
arguments that handle cases involving _FloatN / _FloatNx types as
specified in TS 18661-3 plus some defect fixes.

C2x has further changes to the semantics for <tgmath.h> macros with
such types, which should also be considered defect fixes (although
handled through the integration of TS 18661-3 in C2x rather than
through an issue tracking process).  Specifically, the rules were
changed because of problems raised with using the macros with the
evaluation format types such as float_t and _Float32_t: the older
version of the rules didn't allow passing _FloatN / _FloatNx types to
the narrowing macros returning float or double, or passing float /
double / long double to the narrowing macros returning _FloatN /
_FloatNx, which was a problem with the evaluation format types which
could be either kind of type depending on the value of
FLT_EVAL_METHOD.

Thus the new rules allow cases of mixing types which were not allowed
before - which is not itself a problem for __builtin_tgmath - and, as
part of the changes, the handling of integer arguments was also
changed: if there is any _FloatNx generic argument, integer generic
arguments are treated as _Float32x (not double), while the rule about
treating integer arguments to narrowing macros returning _FloatN or
_FloatNx as _Float64 not double was removed (no longer needed now
double is a valid argument to such macros).

Implement the changes for __builtin_tgmath.  (The changes also added a
rule that if any argument is _DecimalNx, integer arguments are treated
as _Decimal64x, but GCC doesn't support _DecimalNx types so nothing is
done about that.)

I have a corresponding glibc patch to update glibc test expectations
for C2x and also ensure that appropriate semantics are followed when
GCC 7 through 12 are used with <tgmath.h> (avoiding __builtin_tgmath
in cases where it doesn't match the C2x semantics).

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/
	* doc/extend.texi (__builtin_tgmath): Do not restate standard rule
	for handling real integer types.

gcc/c/
	* c-parser.cc (c_parser_postfix_expression): Handle integer
	generic arguments to functions passed to __builtin_tgmath as
	_Float32x if any argument has _FloatNx or _Complex _FloatNx type.
	Do not handle integer arguments to some narrowing functions as
	_Float64.

gcc/testsuite/
	* gcc.dg/builtin-tgmath-3.c: Update expectations and add more
	tests.