clang-format: Improve handling of block comments in braced lists.

Before:
  std::vector<int> v = {
      1, 0 /* comment */
  };

After:
  std::vector<int> v = {1, 0 /* comment */};

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@216445 91177308-0d34-0410-b5e6-96231b3b80d8