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

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

> https://github.com/llvm/llvm-project/pull/89031 should unblock this PR.

@vgvassilev It did. And it uncovered an issue that I didn't think about before: If we set up the executor in the constructor, then the interpreter instance isn't fully initialized yet when we create the LLJIT. Thus, we shouldn't call virtual functions on the interpreter yet (and if we do they will invoke the base-class methods and not the overidden ones). In particular, this breaks the `CreateJITBuilder()` mechanism: https://github.com/llvm/llvm-project/commit/0cf4788d9d0df60980cb48d28aafe7a86aa15a14

For me this patch doesn't have high prio, so I'd probably just drop it and keep everything as is for now. What's your take?

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