[ASTVerifier] Change 'getAs' to 'castTo' when verifying archetypes in

opened pack element environments.

Author: hborla

lib/AST/ASTVerifier.cpp

@@ -645,7 +645,7 @@ class Verifier : public ASTWalker {
             if (genericEnv->getKind() == GenericEnvironment::Kind::OpenedElement) {
               auto contextSubs = genericEnv->getPackElementContextSubstitutions();
               QuerySubstitutionMap isInContext{contextSubs};
-              if (isInContext(root->getInterfaceType()->getAs<GenericTypeParamType>()))
+              if (isInContext(root->getInterfaceType()->castTo<GenericTypeParamType>()))
                 return false;
             }
           }