c++: co_await and initializer_list [PR103871]

When flatten_await_stmt processes the backing array for an initializer_list,
we call cp_build_modify_expr to initialize the promoted variable from the
TARGET_EXPR; that needs to be accepted.

	PR c++/103871
	PR c++/98056

gcc/cp/ChangeLog:

	* typeck.cc (cp_build_modify_expr): Allow array initialization of
	DECL_ARTIFICIAL variable.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/co-await-initlist1.C: New test.