[clang-format] Don't reflow comment lines starting with '@'.

Summary:
This patch stops reflowing comment lines starting with '@', since they commonly
have a special meaning.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29323

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293617 91177308-0d34-0410-b5e6-96231b3b80d8