coroutines: Handle awaiters that are sub-objects [PR95736]

coroutines: Handle awaiters that are sub-objects [PR95736]

Move deciding on initializers for awaitables to the build of the
co_await, this allows us to analyse cases that do not need
a temporary at that point.

As the PR shows, the late analysis meant that we  were not
checking properly for the case that an awaiter is a sub-object
of an existing variable outside the current function scope (and
therefore does not need to be duplicated in the frame).

gcc/cp/ChangeLog:

	PR c++/95736
	* coroutines.cc (get_awaitable_var): New helper.
	(build_co_await): Check more carefully before
	copying an awaitable.
	(expand_one_await_expression): No initializer
	is required when the awaitable is not a temp.
	(register_awaits): Remove handling that is now
	completed when the await expression is built.

gcc/testsuite/ChangeLog:

	PR c++/95736
	* g++.dg/coroutines/pr95736.C: New test.