SystemTools: Implement IsSubDirectory without loop

The loop condition "while ( path.size() > dir.size() )" assumes that
eventually path will be no longer than dir but "c:/" will always be
longer than "/" and loop forever.  Simplify the implementation to avoid
the loop and the assumption.

Change-Id: I5613c7554ebd05b12cfe4882ac3bec4ecaa6e0ae
CMake-Issue: 14283