[clang] [libc++] Re-enable the clang_modules_include test for Objective-C++ (PR #66801)

[Louis Dionne via cfe-commits]

ldionne wrote:

@mordante I'm not certain what's going on here, I get

```
[...]/clang_modules_include.gen.py/__std_clang_module.compile.pass.mm:22:2: error: import of module 'std' imported non C++20 importable modules
   22 | @import std;
      |  ^
1 error generated.
```

Can you assist? It only happens with Clang trunk.


https://github.com/llvm/llvm-project/pull/66801