[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 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 can find arguments in changing the clang-repl target triple to match the loaded module… That would have some reasoning but maybe we won’t be able to start the jit if the triples are too different. 


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