c++: preserve location through constexpr

c++: preserve location through constexpr

While working on the patch for PR101460, I noticed that we were losing the
expression location when folding class prvalue expressions.  The final patch
doesn't fold class prvalues, but this still seems a worthwhile change.  I
don't add location wrappers for scalar prvalues because many callers are
trying to fold them away.

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_outermost_constant_expr): Copy
	expr location to result.