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

[Stefan Gränitz via cfe-commits]

weliveindetail wrote:

@vgvassilev I pushed a quick fix for the tests. I think we should revisit/refactor the test story here though in the mid-term.

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