[clang] Support C++20 Modules in clang-repl (PR #79261)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> For what it's worth, this test appears to fail when `LLVM_DEFAULT_TARGET_TRIPLE` is changed (my script sets it to the output of my Linux distribution's `clang -print-target-triple`)
> 
> ```
> error: PCH file was compiled for the target 'x86_64-pc-linux-gnu' but the current translation unit is being compiled for target 'x86_64-unknown-linux-gnu'
> error: module file .../tools/clang/test/Interpreter/Output/cxx20-modules.cppm.tmp/mod.pcm cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]
> error: Parsing failed.
> ```

Thanks. But I am not sure how should I change the test. Do you have any idea or suggestion? 

https://github.com/llvm/llvm-project/pull/79261