Add a VFSFromYAML class and a parser to create it

Provides a way to create a virtual file system using a YAML file that
supports mapping a file to a path on an 'external' file system. The
external file system will typically be the 'real' file system, but for
testing it can be changed.

A future patch will add a clang option to allow the user to specify such
a file and overlay it, but for now this code is only exercised by the
unit tests.

Differential Revision: http://llvm-reviews.chandlerc.com/D2835