[llvm] Use default instead of a specific case to fix the MSVC warning in sysinfo.cc (PR #149159)

[via llvm-commits]

dyung wrote:

If this will fix the Windows build, can we get this in soon? My Windows bot is still failing and I'd like to get it green again: https://lab.llvm.org/buildbot/#/builders/46

https://github.com/llvm/llvm-project/pull/149159