c++: Fix paren init of aggregates in unevaluated context [PR92947]

c++: Fix paren init of aggregates in unevaluated context [PR92947]

When I implemented C++20 parenthesized initialization of aggregates
I introduced this bogus cp_unevaluated_operand check, thus disabling
this feature in unevaluated context.  Oop.

Removing the check turned up another bug: I wasn't checking the
return value of digest_init.  So when constructible_expr called
build_new_method_call_1 to see if we can construct one type from
another, it got back a bogus INIT_EXPR that looked something like
*(struct T &) 1 = <<< error >>>.  But that isn't the error_mark_node,
so constructible_expr thought we had been successful in creating the
ctor call, and it gave the wrong answer.  Covered by paren-init17.C.

	PR c++/92947 - Paren init of aggregates in unevaluated context.
	* call.c (build_new_method_call_1): Don't check
	cp_unevaluated_operand.  Check the return value of digest_init.

	* g++.dg/cpp2a/paren-init21.C: New test.