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

[Vassil Vassilev via cfe-commits]

vgvassilev 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 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?

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