Summary:
This will be used by ThinLTO to set the amount of backend
parallelism, which performs better when restricted to the number
of physical cores (on X86 at least, where getHostNumPhysicalCores is
currently defined). If not available this falls back to
thread::hardware_concurrency.
Note I didn't add to the thread class since that is a typedef to
std::thread where available.
Reviewers: mehdi_amini
Subscribers: beanz, llvm-commits, mgorny
Differential Revision: https://reviews.llvm.org/D25585
llvm-svn: 284180
