coroutines: Replace extra checks for co_yield with asserts.

coroutines: Replace extra checks for co_yield with asserts.

The lowering of co_yield to a promise method call and a co_await
was moved to the initial analysis phase with the intention of
avoiding the need to handle the two cases later.

Before removing the later checks entirely, this patch replaces
them with checking asserts.

gcc/cp/Changelog:

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

	* coroutines.cc (transform_await_wrapper): Check that we have
	no unlowered co_yields.
	(captures_temporary): Likewise.
	(register_awaits): Likewise.