clang-format: Improve formatting of annotated variables.

Before:
  bool aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa GUARDED_BY(
      aaaaaaaaaaaa) = aaaaaaaa::aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;

After:
  bool aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa GUARDED_BY(aaaaaaaaaaaa) =
      aaaaaaaa::aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206155 91177308-0d34-0410-b5e6-96231b3b80d8