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

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

> Looks like you already implemented it. Nice.

Well, it's gonna be a bit more churn. Eventually, it will reduce coverage for the incremental features in the frontend that are independent from the JIT. To be honest, I don't mind because none of the targets seem relevant for me, just wanted to point that out.

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