For CrashRecoveryContext::RunSafelyOnThread, propagate Darwin's PRIO_DARWIN_BG to the new thread if it is
set on the calling thread.

This allows libclang's indexing threads to propagate their priority to the clang module building threads.

rdar://17459872


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211747 91177308-0d34-0410-b5e6-96231b3b80d8