Lifetime inference: fix a crash on implicit _modify for subscripts.

Fixes rdar://155976839 (Compiler crash when _modify is not specified for
~Escapable type)

Author: atrick

lib/AST/LifetimeDependence.cpp

@@ -1393,7 +1393,7 @@ class LifetimeDependenceChecker {
       return std::nullopt;
     }
     if (wrappedAccessorKind) {
-      auto *var = cast<VarDecl>(accessor->getStorage());
+      auto *var = cast<AbstractStorageDecl>(accessor->getStorage());
       for (auto *wrappedAccessor : var->getAllAccessors()) {
         if (wrappedAccessor->isImplicit())
           continue;