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

[Ilya Biryukov via cfe-commits]

ilya-biryukov 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). I will try to investigate this further today and come back with more details.

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