ThreadLocal: Return a mutable pointer if templated with a non-const type

It makes more sense for ThreadLocal<const T>::get to return a const T*
and ThreadLocal<T>::get to return a T*.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224225 91177308-0d34-0410-b5e6-96231b3b80d8
StringPool: Cleanup typos in unittest comments

No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224226 91177308-0d34-0410-b5e6-96231b3b80d8
ThreadLocal: Move Unix-specific code out of Support/ThreadLocal.cpp

Just a cleanup, no functionality change is intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224227 91177308-0d34-0410-b5e6-96231b3b80d8