[C++20] [Coroutines] Exit early if we found co_await appears in

unevaluated context

Closes https://github.com/llvm/llvm-project/issues/58133

The direct cause for this issue is that the compilation process
continues after it found it is in a invalid state. [expr.await]p2 says
clearly that the co_await expressions are not allowed to appear in
unevaluated context. So we can exit early in this case. It also reduces
many redundant diagnostic messages (Such as 'expression with side
effects has no effect in an unevaluated context').