[ObjC] Bring back the explicit getter for enumDescriptor.

The static analyzer in some versions of Xcode now flag the enumDescriptor_ as
retained by the generated methods and not being release in dealloc. So avoid the
whole issue by just bringing back the explicit getter.

PiperOrigin-RevId: 488436139

objectivec/GPBDescriptor.m

@@ -448,7 +448,6 @@ uint32_t GPBFieldAlternateTag(GPBFieldDescriptor *self) {
 
 @synthesize msgClass = msgClass_;
 @synthesize containingOneof = containingOneof_;
-@synthesize enumDescriptor = enumDescriptor_;
 
 - (instancetype)initWithFieldDescription:(void *)description
                          includesDefault:(BOOL)includesDefault
@@ -623,6 +622,10 @@ uint32_t GPBFieldAlternateTag(GPBFieldDescriptor *self) {
   return enumDescriptor_.enumVerifier(value);
 }
 
+- (GPBEnumDescriptor *)enumDescriptor {
+  return enumDescriptor_;
+}
+
 - (GPBGenericValue)defaultValue {
   // Depends on the fact that defaultValue_ is initialized either to "0/nil" or
   // to an actual defaultValue in our initializer.