Update pddm to work with clang-format

Add clang-format off/on directives around pddm expansions to simplify formatting other code.
diff --git a/objectivec/GPBDictionary.h b/objectivec/GPBDictionary.h
index d00b5b3..28d880d 100644
--- a/objectivec/GPBDictionary.h
+++ b/objectivec/GPBDictionary.h
@@ -45,6 +45,7 @@
 
 //%PDDM-EXPAND DECLARE_DICTIONARIES()
 // This block of code is generated, do not edit it directly.
+// clang-format off
 
 #pragma mark - UInt32 -> UInt32
 
@@ -5478,6 +5479,7 @@
 
 @end
 
+// clang-format on
 //%PDDM-EXPAND-END DECLARE_DICTIONARIES()
 
 NS_ASSUME_NONNULL_END