[FCP] make sure the clang-hash uses relative paths
[FCP] make sure the clang-hash uses relative paths

Summary: Need to use relative paths in order to make the hash consistent across machines

Reviewed By: ngorogiannis

Differential Revision: D23623726

fbshipit-source-id: 44f750658

jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer