c++,coroutines: Handle await expressions in assume attributes.

c++,coroutines: Handle await expressions in assume attributes.

Here we have an expression that is not evaluated but is still seen
as potentially-evaluated.  We handle this by determining if the
operand has side-effects, producing a warning that the assume has
been ignored and eliding it.

gcc/cp/ChangeLog:

	* coroutines.cc (analyze_expression_awaits): Elide assume
	attributes containing await expressions, since these have
	side effects.  Emit a diagnostic that this has been done.

gcc/testsuite/ChangeLog:

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