c++: constexpr bit_cast with empty field

c++: constexpr bit_cast with empty field

The change to only cache constexpr calls that are
reduced_constant_expression_p tripped on bit-cast3.C, which failed that
predicate due to the presence of an empty field in the result of
native_interpret_aggregate, which reduced_constant_expression_p rejects to
avoid confusing output_constructor.

This patch proposes to skip such fields in native_interpret_aggregate, since
they aren't actually involved in the value representation.

gcc/ChangeLog:

	* fold-const.cc (native_interpret_aggregate): Skip empty fields.

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_bit_cast): Check that the result of
	native_interpret_aggregate doesn't need more evaluation.