coroutines: Fix handling of target cleanup exprs [PR94883]

The problem here is that target cleanup expressions have been
added to the initialisers for the awaitable (and returns of
non-trivial values from await_suspend() calls.  This is because
the expansion of the co_await into its control flow is not
apparent to the machinery adding the target cleanup expressions.
The solution being tested is simply to recreate target expressions
as the co_awaits are lowered.  Teaching the machinery to handle
walking co_await expressions in different ways at different points
(outside the coroutine transformation) seems overly complex.

gcc/cp/ChangeLog:

2020-04-30  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94883
	* coroutines.cc (register_awaits): Update target
	expressions for awaitable and suspend handle
	initializers.

gcc/testsuite/ChangeLog:

2020-04-30  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94883
	* g++.dg/coroutines/pr94883-folly-2.C: New test.