[PATCH] Modules fix for relative include paths

[Richard Smith]

  I don't think we can rely on a relative include here. In particular, I don't think that stripping off a `-I` path will necessarily give us a name that will find the same file (considering that one include path might be a prefix of another one, or that we might have used `#include_next` to get to the relevant file). I've committed r203528 instead, which solves this problem by converting the file name to an absolute path.

http://llvm-reviews.chandlerc.com/D3033