coroutines: Wrap co_await in a target expr where needed [PR95050]

coroutines: Wrap co_await in a target expr where needed [PR95050]

Since the co_await expression is mostly opaque to the existing
machinery, we were hiding the details of the await_resume return
value.  If that needs to be wrapped in a target expression, then
emulate this with the whole co_await.  Similarly, if the await
expression we build in response to co_await p.yield_value (e)
is wrapped in a target expression, then we need to transfer that
wrapper to the resultant CO_YIELD_EXPR (which is, itself, just
a proxy for the underlying co_await).

gcc/cp/ChangeLog:

	PR c++/95050
	* coroutines.cc (build_co_await): Wrap the co_await expression
	in a TARGET_EXPR, where needed.
	(finish_co_yield_expr): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/95050
	* g++.dg/coroutines/pr95050.C: New test.