Include Threading.h instead of forward declaring a function.

Previously this led to a circular header dependency, but a recent
change has since removed this dependency, so the correct fix is
to simply include the header rather than forward declare.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211311 91177308-0d34-0410-b5e6-96231b3b80d8