[llvm] r184217 - Add a GetUniqueID that will replace the uniqueID of PathV1.h.

[Rafael Espíndola]

> You can use GetFileInformationByHandle (on an opened file handle) to
> get this -- the volume serial number plus nFileIndexHigh and Low
> uniquely identify the file on a machine.  Note that this identifier
> isn't unique over time (the file system can reuse the ids), so
> hopefully you don't need this for long periods of time.

Patch to improve the windows handling are very welcome, but for the
first patch I just wanted to move from sys::Path with exactly the
semantics we had before.

> ~Aaron

Cheers,
Rafael