Swift: Add db upgrade/downgrade scripts for key-path component extraction.

I've marked both scripts as "partial" since we're extracting different AST components for key-paths and don't have a good way to convert between them in QL. Each deletes the corresponding tables, but leaves non-key-path functionality intact.

swift/downgrades/f937d9e63094280b7ec0ef26c70310daad5c1f79/upgrade.properties

@@ -0,0 +1,8 @@
+description: Use unsafe parsed-path expression instead of component list in keypaths.
+compatibility: partial
+
+key_path_expr_components.rel: delete
+key_path_components.rel: delete
+key_path_component_subscript_arguments.rel: delete
+key_path_component_tuple_indices.rel: delete
+key_path_component_decl_refs.rel: delete

swift/ql/lib/upgrades/e457bc35325b05725c00471da8843f293fe26f8d/upgrade.properties

@@ -0,0 +1,4 @@
+description: Use component list instead of unsafe parsed-path expression in keypaths.
+compatibility: partial
+
+key_path_expr_parsed_paths.rel: delete