[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.

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