c++: constinit and value-initialization [PR119652]

c++: constinit and value-initialization [PR119652]

Value-initialization built an AGGR_INIT_EXPR to set AGGR_INIT_ZERO_FIRST on.
Passing that AGGR_INIT_EXPR to maybe_constant_value returned a TARGET_EXPR,
which potential_constant_expression_1 mistook for a temporary.

We shouldn't add a TARGET_EXPR to the AGGR_INIT_EXPR in this case, just like
we already avoid adding it to CONSTRUCTOR or CALL_EXPR.

	PR c++/119652

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_outermost_constant_expr): Also don't add a
	TARGET_EXPR around AGGR_INIT_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constinit20.C: New test.

(cherry picked from commit c7dc9b6f889fa8f9e4ef060c3af107eaf54265c5)