Fix unused variable in CoroFrame.cpp when building Release with GCC 10

When building with GCC 10, the following warning is reported:

```
/llvm-project/llvm/lib/Transforms/Coroutines/CoroFrame.cpp:1527:28: warning: unused variable ‘CS’ [-Wunused-variable]
 1527 |       if (CatchSwitchInst *CS =
```

This change adds a cast to `void` to avoid the warning.

Reviewed By: lxfind

Differential Revision: https://reviews.llvm.org/D94456

Author: dpaoliello

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -1535,6 +1535,7 @@ static void rewritePHIs(BasicBlock &BB) {
     for (BasicBlock *Pred : Preds) {
       if (CatchSwitchInst *CS =
               dyn_cast<CatchSwitchInst>(Pred->getTerminator())) {
+        (void)CS;
         // CleanupPad with a CatchSwitch predecessor: therefore this is an
         // unwind destination that needs to be handle specially.
         assert(CS->getUnwindDest() == &BB);