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

[Mark de Wever via cfe-commits]

mordante 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.

I tested this locally and indeed it works with clang-17 and fails with clang-18

@ChuanqiXu9 This seems to be introduced in 574ee1c02ef73b66c5957cf93888234b0471695f.

This test uses a Objective-C++ file. Is it easy for you to fix or should I file a bug report?

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