[NFC] Runtime: Mark parameter const.

The implementation doesn't depend on it being non-const and a
forthcoming caller will require it be const.

Author: nate-chandler

stdlib/public/runtime/Metadata.cpp

@@ -4007,7 +4007,7 @@ getSuperclassMetadata(MetadataRequest request, const ClassMetadata *self) {
 
 SWIFT_CC(swift)
 static std::pair<MetadataDependency, const ClassMetadata *>
-getSuperclassMetadata(ClassMetadata *self, bool allowDependency) {
+getSuperclassMetadata(const ClassMetadata *self, bool allowDependency) {
   MetadataRequest request(allowDependency ? MetadataState::NonTransitiveComplete
                                           : /*FIXME*/ MetadataState::Abstract,
                           /*non-blocking*/ allowDependency);