c++: empty union member activation during constexpr [PR102163]

c++: empty union member activation during constexpr [PR102163]

Here, the union's constructor is defined to activate its empty data
member _M_rest, but during constexpr evaluation of this constructor the
subobject constructor call O::O(&_M_rest, 42) doesn't produce a side
effect that actually activates the member, so the union still appears
uninitialized after its constructor has run.  This patch fixes this by
using a dummy MODIFY_EXPR in this situation, whose evaluation ensures
the member gets activated.

	PR c++/102163

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_call_expression): After evaluating a
	subobject constructor call for an empty union member, produce a
	side effect that makes sure the member gets activated.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty17.C: New test.