fix #19304 Borrowing std/times.format causes Error: illformed AST (#20659)

* fix #19304 Borrowing std/times.format causes Error: illformed AST

* follow suggestions

* mitigate for #4121

* improve error message

(cherry picked from commit 2aab03bdfb)

tests/stdlib/t19304.nim

@@ -0,0 +1,7 @@
+import times
+
+type DjangoDateTime* = distinct DateTime
+
+# proc toTime*(x: DjangoDateTime): Time  {.borrow.} # <-- works
+proc format*(x: DjangoDateTime, f: TimeFormat,
+    loc: DateTimeLocale = DefaultLocale): string {.borrow.}