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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > > IncrementalExtensions
> > 
> > 
> > But the change itself is to make `IncrementalExtensions` a compatible lang options. So that we don't need to specify it when generating the modules.
> 
> I mean conditionally if `IncrementalExtensions` is set we change the target triple in the clang fork of the C++ module build.

I am not sure if it is possible since we don't know the clang fork before we see the import. Even if we can, it is more or less problematic if there are multiple imported module from different fork.

> 
> > I am wondering if it helps if we specify the target triple in the test, then it won't be so troublesome.
> 
> I was wondering that too. Can we express that in lit?

For `%clang_cc1`, we can use `%clang_cc1 -triple=x86_64-linux-gnu`. I am not sure about clang-repl. It may be possible by `-Xcc=`.

But due to I can't reproduce the failure, @nathanchance would you like to make this? I don't want to make something that I can't test.

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