clang-format: [proto] Understand text-format nesting without :

Before:
  option (MyProto.options) = {
    field_c : "OK" msg_field{field_d : 123}
  };

After:
  option (MyProto.options) = {
    field_c : "OK"
    msg_field{field_d : 123}
  };

(Note that the colon after "msg_field" is optional).

llvm-svn: 215692