Framework modules cannot have textual headers in umbrella directories

Given:

```
framework module FW {
umbrella "Headers"
module * { export * }
textual header "ignore.h"
}
```

Where the framework’s `Headers` directory contains `ignore.h` and other headers. Clang will still include `ignore.h` in the `` buffer. Non-framework modules do not have this issue.

This causes problems when other parts of the framework include `ignore.h`, as you can get duplicate definitions.

This may be caused by framework modules having more complicated semantics around the module directory. `header` decls are relative to the `Headers` directory, but `umbrella "Headers"` is relative to the framework directory.

Encountered as part of https://github.com/llvm/llvm-project/issues/158249

[testcase.zip](https://github.com/user-attachments/files/22980814/testcase.zip)