[libclang] Introduce libclang APIs for creating a buffer with a JSON virtual file overlay description.

The current API only supports adding 'virtual file path' -> 'real file path' mappings.

rdar://15986708
[SROA] Add a debugging tool which shuffles the slices sequence prior to
sorting it. This helps uncover latent reliance on the original ordering
which aren't guaranteed to be preserved by std::sort (but often are),
and which are based on the use-def chain orderings which also aren't
(technically) guaranteed.

Only available in C++11 debug builds, and behind a flag to prevent noise
at the moment, but this is generally useful so figured I'd put it in the
tree rather than keeping it out-of-tree.