c++: constant direct-initialization [PR108243]

c++: constant direct-initialization [PR108243]

When testing the proposed patch for PR71093 I noticed that it changed the
diagnostic for consteval-prop6.C.  I then noticed that the diagnostic wasn't
very helpful either way; it was complaining about modification of the 'x'
variable, but it's not a problem to initialize a local variable with a
consteval constructor as long as the value is actually constant, we want to
know why the value isn't constant.  And then it turned out that this also
fixed a missed-optimization bug in the testsuite.

	PR c++/108243

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_outermost_constant_expr): Turn
	a constructor CALL_EXPR into a TARGET_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/consteval-prop6.C: Adjust diagnostic.
	* g++.dg/opt/is_constant_evaluated3.C: Remove xfails.