Adjust __builtin_tgmath handling of integer arguments to _FloatN narrowing macros.

Adjust __builtin_tgmath handling of integer arguments to _FloatN narrowing macros.

When adding __builtin_tgmath to support a better tgmath.h
implementation, I noted that further changes might be needed regarding
the TS 18661 functions that round their results to a narrower type,
because of unresolved issues with how the corresponding type-generic
macros are defined in TS 18661.

The resolution of those issues is still in flux, but the latest
version does indeed require something slightly different from
__builtin_tgmath.  It specifies that integer arguments to type-generic
macros such as f32xadd are treated as _Float64 not double - which was
also present in earlier versions of the resolution - but then it also
specifies different handling for _Float64 arguments and double
arguments, which wasn't in earlier versions.  Specifically, in the
latest version
<http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2213.pdf>, f32xadd
with _Float64 arguments would call f32xaddf64, while f32xadd with
double arguments would call f32xaddf64x.  Since integer arguments are
converted directly to the argument type of the selected function (not
to double / _Float64x unless that ends up as the argument type), this
is a user-visible difference in semantics that means __builtin_tgmath
actually needs to implement treating integer arguments as _Float64 in
this case (the rest of the latest semantics can then be implemented in
the header, with a few inline functions there).

To avoid releasing with the older version of the __builtin_tgmath
semantics that doesn't work with the latest proposed DR#13 resolution,
this patch implements a rule in __builtin_tgmath that maps integer
types to _Float64 (respectively _Complex _Float64 for complex integer
types) where all the specified functions return the same _FloatN or
_FloatNx type.  This does not affect any existing uses of
__builtin_tgmath in glibc's or GCC's tgmath.h since I haven't yet
added any of these type-generic macros to glibc when adding the
corresponding narrowing functions.

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

	* doc/extend.texi (__builtin_tgmath): Document when complex
	integer types are treated as _Complex _Float64.

gcc/c:
	* c-parser.c (c_parser_postfix_expression): For __builtin_tgmath
	where all functions return the same _FloatN or _FloatNx type,
	treat integer types as _Float64 instead of double.

gcc/testsuite:
	* gcc.dg/builtin-tgmath-3.c: New test.

From-SVN: r258751