[clang-format] Don't reflow lines starting with TODO, FIXME or XXX.

Summary: These lines commonly carry a special meaning.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29396

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293878 91177308-0d34-0410-b5e6-96231b3b80d8