[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