from a source file and changes clang-check to make use of this.

This makes clang-check just work on in-tree builds, and allows
easy setup via a symlink per source directory to make clang-check
work without any extra configuration.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159990 91177308-0d34-0410-b5e6-96231b3b80d8