[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.

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