[clang-format] Add link to source code in file definitions

Two command line options have been added to clang-doc.
  --repository=<string>       - URL of repository that hosts code; used for links to definition locations.
  --source-root=<string>      - Directory where processed files are stored. Links to definition locations will only be generated if the file is in this dir.

If the file is in the source-root and a repository options is passed;
a link to the source code will be rendered by the HTML generator.

Differential Revision: https://reviews.llvm.org/D65483

llvm-svn: 368460