[AST] Introduce internal attribute '_migration_id'.

It's intended use is to keep track of stdlib changes for migration purposes.

include/swift/Serialization/ModuleFormat.h

@@ -1339,6 +1339,13 @@ namespace decls_block {
                // strings, separated by the prior index
   >;
 
+  using MigrationIdDeclAttrLayout = BCRecordLayout<
+    MigrationId_DECL_ATTR,
+    BCVBR<6>,  // index at the end of the ident,
+    BCBlob     // blob contains the ident and pattern
+               // strings, separated by the prior index
+  >;
+
 #define SIMPLE_DECL_ATTR(X, CLASS, ...) \
   using CLASS##DeclAttrLayout = BCRecordLayout< \
     CLASS##_DECL_ATTR, \