Modernize and simplify HostInfo::GetOSKernelDescription

Replace bool+by-ref argument with llvm::Optional, and move the common
implementation into HostInfoPOSIX. Based on my (simple) experiment,
the uname and the sysctl approach return the same value on MacOS, so
there's no need for a mac-specific implementation of this functionality.

Differential Revision: https://reviews.llvm.org/D112457