Move MessageGenerator::GenerateSource over to Printer::Emit

There are whatespace changes because I couldn't get some of the
generated CPreProcessor things to line up in the exact same way.

PiperOrigin-RevId: 547188309
diff --git a/objectivec/GPBSourceContext.pbobjc.m b/objectivec/GPBSourceContext.pbobjc.m
index cea417d..7ae605b 100644
--- a/objectivec/GPBSourceContext.pbobjc.m
+++ b/objectivec/GPBSourceContext.pbobjc.m
@@ -75,9 +75,9 @@
                                     fieldCount:(uint32_t)(sizeof(fields) / sizeof(GPBMessageFieldDescription))
                                    storageSize:sizeof(GPBSourceContext__storage_)
                                          flags:(GPBDescriptorInitializationFlags)(GPBDescriptorInitializationFlag_UsesClassRefs | GPBDescriptorInitializationFlag_Proto3OptionalKnown | GPBDescriptorInitializationFlag_ClosedEnumSupportKnown)];
-#if defined(DEBUG) && DEBUG
+    #if defined(DEBUG) && DEBUG
       NSAssert(descriptor == nil, @"Startup recursed!");
-#endif  // DEBUG
+    #endif  // DEBUG
     descriptor = localDescriptor;
   }
   return descriptor;