type check AST strings and identifiers

This is related to a21829180423 as well as #12609 and #12610.
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index d6555d6..729ded8 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -794,8 +794,25 @@
     return 0;
 }
 
-#define obj2ast_identifier obj2ast_object
-#define obj2ast_string obj2ast_object
+static int obj2ast_stringlike(PyObject* obj, PyObject** out, PyArena* arena,
+    const char *name)
+{
+    if (!PyUnicode_CheckExact(name)) {
+        PyErr_Format(PyExc_TypeError, "AST %s must be of type str", name);
+        return 1;
+    }
+    return obj2ast_object(obj, out, arena);
+}
+
+static int obj2ast_identifier(PyObject* obj, PyObject** out, PyArena* arena)
+{
+    return obj2ast_stringlike(obj, out, arena, "identifier");
+}
+
+static int obj2ast_string(PyObject* obj, PyObject** out, PyArena* arena)
+{
+    return obj2ast_stringlike(obj, out, arena, "string");
+}
 
 static int obj2ast_int(PyObject* obj, int* out, PyArena* arena)
 {