[ObjC] Multi source mode disables the use of fwd decls.

It isn't worth the effort to try and compute the correct minimal set for each
sub file, and the more Swift folks use on top of this, the less it matters
as the modules will need everything anyways.

PiperOrigin-RevId: 508122902

src/google/protobuf/compiler/objectivec/generator.cc

@@ -281,6 +281,12 @@ bool ObjectiveCGenerator::GenerateAll(
     }
   }
 
+  // Multi source generation forces off the use of fwd decls in favor of
+  // imports.
+  if (generation_options.experimental_multi_source_generation) {
+    generation_options.headers_use_forward_declarations = false;
+  }
+
   // -----------------------------------------------------------------
 
   // These are not official generation options and could be removed/changed in