coroutines: Ensure co_await_exprs have TREE_SIDE_EFFECTS set [PR 101133].

coroutines: Ensure co_await_exprs have TREE_SIDE_EFFECTS set [PR 101133].

Although it is not immediately evident from the symptoms, the PR is
caused by a variable having a DECL_INITIAL() containing a co_await.
This is not correct, since such expressions have side-effects.

We were marking the overall co_await expression correctly, but if a
consumer of that expression stripped out the underlying co_await_expr
then the latter would not be properly marked.

Fixed by marking both the underlying and any containing await expr
with TREE_SIDE_EFFECTS.  Also mark type-dependent co_await expressions.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
	PR c++/101133

gcc/cp/ChangeLog:

	* coroutines.cc (build_co_await): Mark co_await_expr trees
	with TREE_SIDE_EFFECTS, also mark any containing expression.
	(finish_co_await_expr): Mark type-dependent co_await_expr
	trees with TREE_SIDE_EFFECTS.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/pr101133.C: New test.