c++: maybe_constant_init and unevaluated operands [PR109030]

c++: maybe_constant_init and unevaluated operands [PR109030]

This testcase in this PR (already fixed by r13-6526-ge4692319fd5fc7)
demonstrates that maybe_constant_init can be called on an unevaluated
operand (e.g. from massage_init_elt) so this entry point should also
limit constant evaluation in that case, like maybe_constant_value does.

	PR c++/109030

gcc/cp/ChangeLog:

	* constexpr.cc (maybe_constant_init_1): For an unevaluated
	non-manifestly-constant operand, don't constant evaluate
	and instead call fold_to_constant as in maybe_constant_value.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-inst2.C: New test.