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