coroutines: Update TREE_SIDE_EFFECTS on inserted bind exprs.

coroutines: Update TREE_SIDE_EFFECTS on inserted bind exprs.

There are several places where we insert bind expressions while
making the coroutine AST transforms.  These should be marked as
having side-effects where relevant, which had been omitted.  This
leads to at least one failure in the cppcoros test suite, where a loop
body is dropped in gimplification because it is not marked.

gcc/cp/ChangeLog:

2020-05-10  Iain Sandoe  <iain@sandoe.co.uk>

	Backported from mainline
	2020-05-08  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/95003
	* coroutines.cc (build_actor_fn): Ensure that bind scopes
	are marked as having side-effects where necessary.
	(replace_statement_captures): Likewise.
	(morph_fn_to_coro): Likewise.

gcc/testsuite/ChangeLog:

2020-05-10  Iain Sandoe  <iain@sandoe.co.uk>

	Backported from mainline
	2020-05-08  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/95003
	* g++.dg/coroutines/torture/pr95003.C: New test.