[misc-coroutine-hostile-raii] Use getOperand instead of getCommonExpr. (#79206)

We were previously allowlisting awaitable types returned by
`await_transform` instead of the type of the operand of the `co_await`
expression.

This previously used to give false positives and not respect the
`AllowedAwaitablesList` flag when `await_transform` is used. See added
test cases for such examples.