clang-format: Allow formatting short enums on a single line.

Before:
  enum ShortEnum {
    A,
    B,
    C
  };

After:
  enum ShortEnum { A, B, C };

This seems to be the predominant choice in LLVM/Clang as well as in
Google style.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198558 91177308-0d34-0410-b5e6-96231b3b80d8