[ConstraintSystem] Add a missing case to locator simplification

If tuple element is anchored on an assignment, we need to look
at the source of the assignment to find tuple expression the
element belongs to.

(cherry picked from commit 557c6f7)

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -5705,6 +5705,13 @@ void constraints::simplifyLocator(ASTNode &anchor,
       // Extract tuple element.
       auto elt = path[0].castTo<LocatorPathElt::AnyTupleElement>();
       unsigned index = elt.getIndex();
+
+      if (auto *AE = getAsExpr<AssignExpr>(anchor)) {
+        if (isa<TupleExpr>(AE->getSrc())) {
+          anchor = AE->getSrc();
+        }
+      }
+
       if (auto tupleExpr = getAsExpr<TupleExpr>(anchor)) {
         if (index < tupleExpr->getNumElements()) {
           anchor = tupleExpr->getElement(index);
@@ -5720,6 +5727,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
           continue;
         }
       }
+
       break;
     }