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

include/llvm/Support/FileSystem.h

@@ -580,7 +580,10 @@ enum OpenFlags {
 
   /// F_Binary - The file should be opened in binary mode on platforms that
   /// make this distinction.
-  F_Binary = 4
+  F_Binary = 4,
+
+  /// Open the file for read and write.
+  F_RW = 8
 };
 
 inline OpenFlags operator|(OpenFlags A, OpenFlags B) {