Share a createUniqueEntity implementation between unix and windows.

The only extra bit of functionality that had to be exposed for this be be
implemented in Path.cpp is opening a file in rw mode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202005 91177308-0d34-0410-b5e6-96231b3b80d8