clang-format: Improve braced-list detection.

Before:
  vector<int> v { 12 }
      GUARDED_BY(mutex);

After:
  vector<int> v{12} GUARDED_BY(mutex);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289525 91177308-0d34-0410-b5e6-96231b3b80d8