c++: fix wrong-code with constexpr prvalue opt [PR118396]

c++: fix wrong-code with constexpr prvalue opt [PR118396]

The recent r15-6369 unfortunately caused a bad wrong-code issue.
Here we have

  TARGET_EXPR <D.2996, (void) (D.2996 = {.status=0, .data={._vptr.Foo=&_ZTV3Foo + 16}})>

and call cp_fold_r -> maybe_constant_init with object=D.2996.  In
cxx_eval_outermost_constant_expr we now take the type of the object
if present.  An object can't have type 'void' and so we continue to
evaluate the initializer.  That evaluates into a VOID_CST, meaning
we disregard the whole initializer, and terrible things ensue.

For non-simple TARGET_EXPRs, we should return ctx.ctor rather than
the result of cxx_eval_constant_expression.

	PR c++/118396
	PR c++/118523

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_outermost_constant_expr): For non-simple
	TARGET_EXPRs, return ctx.ctor rather than the result of
	cxx_eval_constant_expression.  If TYPE and the type of R don't
	match, return the original expression.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-prvalue4.C: New test.
	* g++.dg/cpp1y/constexpr-prvalue3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>