[C++] constexpr: request insert iff depth is ok

[C++] constexpr: request insert iff depth is ok

cxx_eval_call_expression requests an INSERT even in cases when it
would later decide not to insert.  This could break double-hashing
chains.  Arrange for it to use NO_INSERT when the insertion would not
be completed.

for  gcc/cp/ChangeLog

	* constexpr.cc (cxx_eval_call_expression): Do not request an
	INSERT that would not be completed.