Fix include path in ClangTidy.cpp.

Summary:
https://llvm.org/bugs/show_bug.cgi?id=27355
To compile with other binary output directory structures in build systems like Android.

Reviewers: srhines, alexfh

Subscribers: tberghammer, danalbert, cfe-commits

Differential Revision: http://reviews.llvm.org/D19249

llvm-svn: 267835