clang-format: [Proto] Handle enum bodies differently.

In proto, enum constants can contain complex options and should be
handled more like individual declarations.

Before:
  enum Type {
    UNKNOWN = 0 [(some_options) =
                     {
                       a: aa,
                       b: bb
                     }];
  };

After:
  enum Type {
    UNKNOWN = 0 [(some_options) = {
      a: aa,
      b: bb
    }];
  };

llvm-svn: 242404