coroutines: Don't make duplicate frame copies of awaitables.

coroutines: Don't make duplicate frame copies of awaitables.

In general, we need to manage the lifetime of compiler-
generated awaitable instances in the coroutine frame, since
these must persist across suspension points.

However, it is quite possible that the user might provide the
awaitable instances, either as function params or as a local
variable.  We will already generate a frame entry for these as
required.

At present, under this circumstance, we are duplicating these,
awaitable, initialising a second frame copy for them (which we
then subsequently destroy manually after the suspension point).
That's not efficient - so an undesirable thinko in the first place.
However, there is also an actual bug; if the compiler elects to
elide the copy (which is perfectly legal), it does not have visibility
of the manual management of the post-suspend destruction
- this subsequently leads to double-free errors.

The solution is not to make the second copy (as noted, params
and local vars already have frame copies with managed lifetimes).

gcc/cp/ChangeLog:

2020-03-02  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (build_co_await): Do not build frame
	proxy vars when the co_await expression is a function
	parameter or local var.
	(co_await_expander): Do not initialise a frame var with
	itself.
	(transform_await_expr): Only substitute the awaitable
	frame var if it's needed.
	(register_awaits): Do not make frame copies for param
	or local vars that are awaitables.

gcc/testsuite/ChangeLog:

2020-03-02  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/torture/func-params-09-awaitable-parms.C: New test.
	* g++.dg/coroutines/torture/local-var-5-awaitable.C: New test.