[CSSimplify] Allow any Sendable -> Any if subscript appears in preconcurrency context

(cherry picked from commit b7a7839)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -3722,6 +3722,10 @@ static bool matchSendableExistentialToAnyInGenericArgumentPosition(
             return isPreconcurrencyContext(
                 cs.getConstraintLocator(UDE->getBase()));
           }
+          if (auto *SE = getAsExpr<SubscriptExpr>(calleeLoc->getAnchor())) {
+            return isPreconcurrencyContext(
+                cs.getConstraintLocator(SE->getBase()));
+          }
           return false;
         }