[clang] [clang-repl] Expose CreateExecutor() and ResetExecutor() in extended Interpreter interface (PR #84460)

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

> > Do you mind incorporating this patch so that we avoid churn?
> 
> Sure. Essentially, this drops lazy creation of the executor and makes it dependent on the frontend action explicitly. Fine for me. We can still expose explicit setup/tear-down.

@vgvassilev I moved it into the separate review https://github.com/llvm/llvm-project/pull/84758, because it has unexpected side-effects.

https://github.com/llvm/llvm-project/pull/84460