[NFC] Make it illegal to reset the interface type to null

Author: CodaFi

lib/AST/Decl.cpp

@@ -2940,6 +2940,7 @@ Type ValueDecl::getInterfaceType() const {
 }
 
 void ValueDecl::setInterfaceType(Type type) {
+  assert(!type.isNull() && "Resetting the interface type to null is forbidden");
   getASTContext().evaluator.cacheOutput(InterfaceTypeRequest{this},
                                         std::move(type));
 }