[llvm] Fix MSVC warning in benchmark (PR #147357)

[Peter Klausler via llvm-commits]

klausler wrote:

```
4.170 [148/4/9] Building CXX object third-party/benchmark/src/CMakeFiles/benchmark.dir/sysinfo.cc.obj
FAILED: third-party/benchmark/src/CMakeFiles/benchmark.dir/sysinfo.cc.obj 
C:\Users\tcwg\scoop\shims\ccache.exe C:\Users\tcwg\scoop\apps\llvm-arm64\current\bin\clang-cl.exe  /nologo -TP -DBENCHMARK_STATIC_DEFINE -DEXPERIMENTAL_KEY_INSTRUCTIONS -DHAVE_STD_REGEX -DHAVE_STEADY_CLOCK -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_GLIBCXX_ASSERTIONS -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -IC:/Users/tcwg/llvm-worker/flang-arm64-windows-msvc/build/third-party/benchmark/src -IC:/Users/tcwg/llvm-worker/flang-arm64-windows-msvc/llvm-project/third-party/benchmark/src -IC:/Users/tcwg/llvm-worker/flang-arm64-windows-msvc/build/include -IC:/Users/tcwg/llvm-worker/flang-arm64-windows-msvc/llvm-project/llvm/include -IC:/Users/tcwg/llvm-worker/flang-arm64-windows-msvc/llvm-project/third-party/benchmark/include /DWIN32 /D_WINDOWS   /Zc:inline /Zc:__cplusplus /Oi /Brepro /bigobj /permissive- -Werror=unguarded-availability-new   -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported /Gw /W4  -EHs-  -EHa- /O2 /Ob2  -std:c++14 -MD -UNDEBUG /showIncludes /Fothird-party/benchmark/src/CMakeFiles/benchmark.dir/sysinfo.cc.obj /Fdthird-party\benchmark\src\CMakeFiles\benchmark.dir\benchmark.pdb -c -- C:/Users/tcwg/llvm-worker/flang-arm64-windows-msvc/llvm-project/third-party/benchmark/src/sysinfo.cc
C:/Users/tcwg/llvm-worker/flang-arm64-windows-msvc/llvm-project/third-party/benchmark/src/sysinfo.cc(374,12): error: use of undeclared identifier 'CacheUnknown'
  374 |       case CacheUnknown:
      |            ^
1 error generated.
```

https://github.com/llvm/llvm-project/pull/147357