[ObjC] Tweak some things to be better shape for clang-format.

- Minor formatting changes to make thing happy.
- Block clang-format from the PDDM macro definitions to avoid it wrapping
  things.
- Don't add clang-format directives to the expansion, easier to handling
  it outside of there.

objectivec/GPBProtocolBuffers_RuntimeSupport.h

diff --git a/objectivec/GPBProtocolBuffers_RuntimeSupport.h b/objectivec/GPBProtocolBuffers_RuntimeSupport.h
index 04dde62..804a68a 100644
--- a/objectivec/GPBProtocolBuffers_RuntimeSupport.h
+++ b/objectivec/GPBProtocolBuffers_RuntimeSupport.h
@@ -31,7 +31,9 @@
 // This header is meant to only be used by the generated source, it should not
 // be included in code using protocol buffers.
 
+// clang-format off
 #import "GPBBootstrap.h"
+// clang-format on
 
 #import "GPBDescriptor_PackagePrivate.h"
 #import "GPBExtensionInternals.h"