clang-format: Support ObjC's NS_ENUMs.

Before:
  typedef NS_ENUM(NSInteger, MyType) {
      /// Information about someDecentlyLongValue.
      someDecentlyLongValue,
      /// Information about anotherDecentlyLongValue.
      anotherDecentlyLongValue,
      /// Information about aThirdDecentlyLongValue.
      aThirdDecentlyLongValue};

After:
  typedef NS_ENUM(NSInteger, MyType) {
    /// Information about someDecentlyLongValue.
    someDecentlyLongValue,
    /// Information about anotherDecentlyLongValue.
    anotherDecentlyLongValue,
    /// Information about aThirdDecentlyLongValue.
    aThirdDecentlyLongValue
  };

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200469 91177308-0d34-0410-b5e6-96231b3b80d8