cp+coroutines: teach convert_to_void to diagnose discarded co_awaits

cp+coroutines: teach convert_to_void to diagnose discarded co_awaits

co_await expressions are nearly calls to Awaitable::await_resume, and,
as such, should inherit its nodiscard.  A discarded co_await expression
should, hence, act as if its call to await_resume was discarded.

This patch teaches convert_to_void how to discard 'through' a
CO_AWAIT_EXPR. When we discard a CO_AWAIT_EXPR, we can also just discard
the await_resume() call conveniently embedded within it.  This results
in a [[nodiscard]] diagnostic that the PR noted was missing.

gcc/cp/ChangeLog:

	PR c++/110171
	* coroutines.cc (co_await_get_resume_call): New function.
	Returns the await_resume expression of a given co_await.
	* cp-tree.h (co_await_get_resume_call): New function.
	* cvt.cc (convert_to_void): Handle CO_AWAIT_EXPRs and call
	maybe_warn_nodiscard on their resume exprs.

gcc/testsuite/ChangeLog:

	PR c++/110171
	* g++.dg/coroutines/pr110171-1.C: New test.
	* g++.dg/coroutines/pr110171.C: New test.