[clang] Make -frewrite-includes put an endif at the end of the included text (PR #67613)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

>From what I can tell, this looks correct, though I don't know much about this rewriter, I've only spent a little time reviewing it.  That said, i think this is a very nice improvement.

One thing I might request (in addition to what I did in teh comments :D) is a similar change in `-frewrite-modules`.

https://github.com/llvm/llvm-project/pull/67613