[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:

```
error: 'expected-error' diagnostics seen but not expected:
  File /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-6mhbj-1/llvm-project/clang-ci/clang/test/Modules/explicit-build-overlap.cpp Line 11: module use does not directly depend on a module exporting 'a.h', which is part of indirectly-used module b
1 error generated.
```

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