[clang] [clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds (PR #83660)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> It's definitely caused by that change, reverting it fixes the failure.
> 
> We use module maps so that some of our `#include` get processed as `#import`, I believe the same code gets executed for our use-case. Because our setup is so unusual, producing a repro is hard and make take some time. (Build system generated module maps and `#include` is essentially turned into `#import` by Clang).

clang never transforms `#include` to `#import`. It will effectively attempt to transform `#include` and `#import` to `@import` but that's different.

https://github.com/llvm/llvm-project/pull/83660