c++: Handle std::construct_at on automatic vars during constant evaluation [PR97195]

c++: Handle std::construct_at on automatic vars during constant evaluation [PR97195]

As mentioned in the PR, we only support due to a bug in constant expressions
std::construct_at on non-automatic variables, because we VERIFY_CONSTANT the
second argument of placement new, which fails verification if it is an
address of an automatic variable.
The following patch fixes it by not performing that verification, the
placement new evaluation later on will verify it after it is dereferenced.

2020-10-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/97195
	* constexpr.c (cxx_eval_call_expression): Don't VERIFY_CONSTANT the
	second argument.

	* g++.dg/cpp2a/constexpr-new14.C: New test.