Fix bug 20892 - clang-format does not handle C-style comments

Summary:
http://llvm.org/bugs/show_bug.cgi?id=20892

Add support of C-style formatting enabling/disabling directives. Now the following two styles are supported:

  // clang-format on
  /* clang-format on */

The flexibility in comments (support of extra spaces and/or slashes, etc.) is deliberately avoided to simplify search in large code bases.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, curdeius, klimek

Differential Revision: http://reviews.llvm.org/D5309