r215853 - Convert a few ownership comments with std::unique_ptr.

[Rafael Espíndola]

> Directly constructing temporary smart pointers is an eye-brow raising
> thing (though not applicable to LLVM - it's not exception safe). I'd
> probably be inclined to take ownership on the line before (with the
> result of getMemBuffer) into a named unique_ptr, then move into the
> call got getVFSFromYAML instead.

Fixed in r215865.

> (& perhaps one day we'll have MemoryBuffer's factories return by
> unique_ptr... -

That is where I am going :-)

Cheers,
Rafael