[clang] [clang][modules] HeaderSearch::MarkFileModuleHeader creates extra HeaderFileInfo, breaks PCM reuse (PR #89005)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Also note that `ASTWriter` uses this logic in couple of places to avoid serializing unrelated headers:

```c++
if (!HFI || (HFI->isModuleHeader && !HFI->isCompilingModuleHeader))
  continue;
```

Since textual headers from other modules have `isModuleHeader=false` and `isCompilingModuleHeader=false` after #83660 we always serialize them, even if we just implicitly found their module map and never entered them. I didn't check how this patch interacts with that logic, just wanted to surface this.

https://github.com/llvm/llvm-project/pull/89005