[clang] [clang-repl] Set up executor implicitly to account for init PTUs (PR #84758)

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

Taking this patch would mean that deriving from Interpreter won't hold as the mechanism for extensions! At least for customizing the JITBuilder. I propose to pass the `LLJITBuilder` to the constructor then and to store it as a member for later `Create/ResetExecutor()` calls. Let me prototype that..

https://github.com/llvm/llvm-project/pull/84758