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

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

FYI: The run on the Windows bot failed even though tests worked as expected. In particular, all `ClangReplInterpreterTests` unittests passed as well as all `Interpreter` LIT tests. I could land this towards the end of the week.

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