Merge TempDir and system_temp_directory.

We had two functions for finding the temp or cache directory. Each had a
different set of smarts about OS specific APIs.

With this patch system_temp_directory becomes the only way to do it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216460 91177308-0d34-0410-b5e6-96231b3b80d8