c++: Tolerate cdtors returning this in constexpr

c++: Tolerate cdtors returning this in constexpr

On targets that return this from cdtors, cxx_eval_call_expression may
flag flowing off the end of a dtor.  That's preempted for ctors, and
avoided entirely when dtors return void, but when they return this,
the return value should be conceptually disregarded, without making
room for such internal ABI details to make a program ill-formed, as in
g++.dg/cpp2a/constexpr-dtor12.C on arm-eabi.

for  gcc/cp/ChangeLog

	* constexpr.cc (cxx_eval_call_expression): Disregard dtor
	result.