clang-format: Let a trailing comma in braced lists enforce linebreaks.

Before:
  vector<int> x{1, 2, 3, 4, };

After:
  vector<int> x{
      1, 2, 3, 4,
  };

This fixes llvm.org/PR18519.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204458 91177308-0d34-0410-b5e6-96231b3b80d8