Fix MSVC warning in benchmark (#147357)
Building LLVM with MSVC is raising the following warning:
```
llvm\third-party\benchmark\src\sysinfo.cc(375): warning C4062: enumerator 'CacheUnknown' in switch of enum '_PROCESSOR_CACHE_TYPE' is not handled
```
This change resolves the warning by moving the `Unknown` type into a
case block for `CacheUnknown`.
Not sure how this code flows back into the original source.
