[AST] Avoid possible segfault in isDirectToStorageAccess
`isAccessibleFrom` allows a `nullptr` for `useDC`. Thus `UseDC` can be a
`nullptr` in the path `checkAccess` -> `getAccessSemanticsFromContext`
-> `isDirectToStorageAccess`.
Resolves rdar://104620331.
(cherry picked from commit 526016c5c1e8748e3aca78628c5d1a00b9d83666)
