[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:

Also this change isn't trying to modify `#include` in any kind of way, _only_ `#import`. So it's quite intentional that your test behaves the same before and after.

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