[clang] [Modules] textual headers in submodules never resolve their `use`s (PR #69651)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

LGTM. The rest of clang reasons about this at the top level module, so so should this.

https://github.com/llvm/llvm-project/pull/69651