Complete Header Change Description Reading functionality

Header change description files are now discovered in the repo. From
these files alternate versions of headers are created and placed in
branches for merging. As few branches as possible are created.

Other fixes:
* offsets and lengths in some yaml files fixed.
* LLVM doesn't support multi-line literals so using quotes and \n
  instead.
* Added a script fixFilenames.sh to fix the filenames in the yaml files
  to be absolute paths.
* Now link against libclang.so to gain accessing to Rewriter and Tooling
  functionality.