[llvm][Support] Provide interface to set thread priorities

Summary:
We have a multi-platform thread priority setting function(last piece
landed with D58683), I wanted to make this available to all llvm community,
there seem to be other users of such functionality with portability fixmes:
lib/Support/CrashRecoveryContext.cpp
tools/clang/tools/libclang/CIndex.cpp

Reviewers: gribozavr, ioeric

Subscribers: krytarowski, jfb, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59130

llvm-svn: 358494