Add a generation option to control use of forward declarations in headers.

Swift importing ObjC drops methods/properties if the type is only a forward
declaration since the type is incomplete. Historically the generator has always
use forward declarations to reduce how much will have rebuild when a proto file
does change; but that puts it at odds with Swift. If ObjC Protos end up spanning
Swift modules, the Swift import behavior could become a problem; so this option
provides a control for the behavior. The current behavior is to continue forward
declarations, but eventually the default will be changed.

Generate the WKTs using imports instead of forward decls.

objectivec/GPBType.pbobjc.h

diff --git a/objectivec/GPBType.pbobjc.h b/objectivec/GPBType.pbobjc.h
index ee02b0a..b023050 100644
--- a/objectivec/GPBType.pbobjc.h
+++ b/objectivec/GPBType.pbobjc.h
@@ -4,6 +4,8 @@
 #import "GPBDescriptor.h"
 #import "GPBMessage.h"
 #import "GPBRootObject.h"
+#import "GPBAny.pbobjc.h"
+#import "GPBSourceContext.pbobjc.h"
 
 #if GOOGLE_PROTOBUF_OBJC_VERSION < 30004
 #error This file was generated by a newer version of protoc which is incompatible with your Protocol Buffer library sources.
@@ -19,11 +21,9 @@
 
 CF_EXTERN_C_BEGIN
 
-@class GPBAny;
 @class GPBEnumValue;
 @class GPBField;
 @class GPBOption;
-@class GPBSourceContext;
 
 NS_ASSUME_NONNULL_BEGIN