runtime: silence -Wunused-variable warning (NFC)

`classMetadata` is only used in the ObjC path, resulting in a
`-Wunused-variable` warning.  Sink the variable into the ObjC path.
Because the conversion of the metadata does not rely on the type
metadata bits in the metadata, it is safe to delay the definition and
bit adjustment to the point where it is used unifying the two ObjC
paths.

Author: compnerd

stdlib/public/runtime/Metadata.cpp

@@ -113,15 +113,14 @@ Metadata *TargetSingletonMetadataInitialization<InProcess>::allocate(
   // Such a template will fail the `isTypeMetadata` test and we'll think that it
   // isn't Swift metadata but a plain old ObjC class instead.
   if (metadata->getKind() == MetadataKind::Class) {
-    auto *classMetadata = static_cast<ClassMetadata*>(metadata);
-#if SWIFT_OBJC_INTEROP
-    classMetadata->setAsTypeMetadata();
-#endif
     auto *fullMetadata = asFullMetadata(metadata);
 
     // Begin by initializing the value witness table; everything else is
     // initialized by swift_initClassMetadata().
 #if SWIFT_OBJC_INTEROP
+    auto *classMetadata = static_cast<ClassMetadata*>(metadata);
+    classMetadata->setAsTypeMetadata();
+
     fullMetadata->ValueWitnesses =
       (classMetadata->Flags & ClassFlags::UsesSwiftRefcounting)
          ? &VALUE_WITNESS_SYM(Bo)