[Clang][SemaCXX][Coroutines] Fix misleading diagnostics with -Wunsequenced

D115187 exposed CoroutineSuspendExpr's operand, which makes some nodes to show
up twice during the traversal, confusing the check for unsequenced operations.
Skip the operand since it's already handled as part of the common expression
and get rid of the misleading warnings.

https://github.com/llvm/llvm-project/issues/56768

Differential Revision: https://reviews.llvm.org/D142077