[CSSimplify] NFC: Straighten checking of type variable passed to resolvePackExpansion

The type variable should always be one for pack expansion type which
implies that locator always ends at `PackExpansionType`.

(cherry picked from commit 043d2aa)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -11492,16 +11492,13 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
 
 bool ConstraintSystem::resolvePackExpansion(TypeVariableType *typeVar,
                                             Type contextualType) {
-  auto *locator = typeVar->getImpl().getLocator();
+  assert(typeVar->getImpl().isPackExpansion());
 
-  Type openedExpansionType;
-  if (auto expansionElt =
-          locator->getLastElementAs<LocatorPathElt::PackExpansionType>()) {
-    openedExpansionType = expansionElt->getOpenedType();
-  }
+  auto *locator = typeVar->getImpl().getLocator();
 
-  if (!openedExpansionType)
-    return false;
+  Type openedExpansionType =
+      locator->castLastElementTo<LocatorPathElt::PackExpansionType>()
+          .getOpenedType();
 
   assignFixedType(typeVar, openedExpansionType, locator);