Fix tgmath.h for bit-fields (bug 21685).

The tgmath.h macros produce errors for bit-field arguments, because
they apply sizeof and typeof to the arguments.  This patch fixes them
to use unary + systematically before using sizeof or typeof on
arguments that might be bit-fields (note that __real__ of a bit-field
is still a bit-field for this purpose, since it's an lvalue).
gen-tgmath-tests.py is extended to add tests for this case.

Tested for x86_64.

	[BZ #21685]
	* math/tgmath.h (__tgmath_real_type): Use unary + on potentially
	bit-field expressions passed to sizeof or typeof.
	[__HAVE_FLOAT128 && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(__TGMATH_F128): Likewise.
	[__HAVE_FLOAT128 && __GLIBC_USE (IEC_60559_TYPES_EXT)]
	(__TGMATH_CF128): Likewise.
	(__TGMATH_UNARY_REAL_ONLY): Likewise.
	(__TGMATH_UNARY_REAL_RET_ONLY): Likewise.
	(__TGMATH_BINARY_FIRST_REAL_ONLY): Likewise.
	(__TGMATH_BINARY_FIRST_REAL_STD_ONLY): Likewise.
	(__TGMATH_BINARY_REAL_ONLY): Likewise.
	(__TGMATH_BINARY_REAL_STD_ONLY): Likewise.
	(__TGMATH_BINARY_REAL_RET_ONLY): Likewise.
	(__TGMATH_TERNARY_FIRST_SECOND_REAL_ONLY): Likewise.
	(__TGMATH_TERNARY_REAL_ONLY): Likewise.
	(__TGMATH_TERNARY_FIRST_REAL_RET_ONLY): Likewise.
	(__TGMATH_UNARY_REAL_IMAG): Likewise.
	(__TGMATH_UNARY_IMAG): Likewise.
	(__TGMATH_UNARY_REAL_IMAG_RET_REAL): Likewise.
	(__TGMATH_BINARY_REAL_IMAG): Likewise.
	* math/gen-tgmath-tests.py (Type.init_types): Create bit_field
	type.
	(define_vars_for_type): Handle bit_field type specially.
	(Tests.__init__): Declare structure with bit-field element.