Add a higher level createTemporaryFile function.

This function is inspired by clang's Driver::GetTemporaryPath. It hides the
pattern used for uniquing and requires simple file names that are always
placed in the system temporary directory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185716 91177308-0d34-0410-b5e6-96231b3b80d8