[llvm-bugs] [Bug 132792] Re-exporting a module in a modulemap does not seem to work

[LLVM Bugs via llvm-bugs]

Issue	132792
Summary	Re-exporting a module in a modulemap does not seem to work
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Reporter	Steelskin

    ### Summary

Consider the following modulemap file:
```
module Interop {
    header "interop.h"
    export *
}

module Container {
    export Interop
}
```

I would expect that using the `Container` module would contain all of the definitions from the `Interop` module, as the [documentation](https://clang.llvm.org/docs/Modules.html#export-declaration) seems to indicate:
> An export-declaration specifies which imported modules will automatically be re-exported as part of a given module’s API.

However, I have found that this does not work as expected.

### Repro steps

```
// module.modulemap
module Interop {
    header "interop.h"
    export *
}

module Container {
    export Interop
}
```

```c++
// interop.h
# pragma once

int g_property = 0;
```

Build commands:
```bash
clang -cc1 -emit-module -o Container.pcm -fmodules module.modulemap -fmodule-name=Container
clang -cc1 -emit-module -o Interop.pcm -fmodules module.modulemap -fmodule-name=Interop
```

I could not figure an easy way to get the symbols out of the PCM, but I can infer the lack of `g_property` symbol in the `Container` module in 2 ways:

1. `strings` does not find `g_property` in `Interop.pcm`
```bash
$ strings Container.pcm | grep g_property
$ string Interop.pcm | grep g_property

g_property
```

2. `clang -module-file-info` does not list `interop.h` for `Container.pcm`
```bash
$ clang -module-file-info Container.pcm
[...]
    Use libc++ (rather than libstdc++) [-stdlib=]: No
  Preprocessor options:
    Uses compiler/target-specific predefines [-undef]: Yes
    Uses detailed preprocessing record (for indexing): No
    Predefined macros:
  Input file: [..]/module.modulemap
  Diagnostic options:
[...]
$ clang -module-file-info Interop.pcm
[...]
    Use libc++ (rather than libstdc++) [-stdlib=]: No
  Preprocessor options:
    Uses compiler/target-specific predefines [-undef]: Yes
    Uses detailed preprocessing record (for indexing): No
    Predefined macros:
  Input file: [..]/module.modulemap
  Input file: [..]/interop.h
  Diagnostic options:
[...]
```

### Related issue

This is problematic when using modulemaps with Swift, see swiftlang/swift#80216

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