[ConstraintSystem] Fix locator builder directlyAt

`hasEmptyPath` in locator builder context means that there are
no new elements added which is different from the materialized
locator, `directlyAt` has to use `!last()` to check whether or
not there are any elements in the path of this builder.

Author: xedin

include/swift/Sema/ConstraintLocator.h

@@ -1264,11 +1264,11 @@ class ConstraintLocatorBuilder {
   template <typename E>
   bool directlyAt() const {
     if (auto *expr = getAnchor().dyn_cast<Expr *>())
-      return isa<E>(expr) && hasEmptyPath();
+      return isa<E>(expr) && !last();
     return false;
   }
 
-  /// Determine whether this builder has an empty path.
+  /// Determine whether this builder has an empty path (no new elements).
   bool hasEmptyPath() const {
     return !element;
   }