SystemTools: Teach GetActualCaseForPath to convert as much as possible

If only some leading components of the input path exist then convert
those and leave the rest unchanged.  This way if a file path is
constructed using GetActualCaseForPath and created on disk, then a
following GetActualCaseForPath on the same now-existing path will return
the same value.

Change-Id: I6e09a4d411f8bf107312f39d7409ba2d22bf176b
CMake-Issue: 16295