coroutines : Adjust constraints on when to build ctors [PR98118].

coroutines : Adjust constraints on when to build ctors [PR98118].

PR98118 shows that TYPE_NEEDS_CONSTRUCTING is necessary but not
sufficient.  Use type_build_ctor_call() instead.

gcc/cp/ChangeLog:

	PR c++/98118
	* coroutines.cc (build_co_await): Use type_build_ctor_call()
	to determine cases when a CTOR needs to be built.
	(flatten_await_stmt): Likewise.
	(morph_fn_to_coro): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/98118
	* g++.dg/coroutines/pr98118.C: New test.