[clang] [clang-repl] Rework layering of incremental executors. (PR #175448)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> Let's merge this and then have this diff ([#175448 (comment)](https://github.com/llvm/llvm-project/pull/175448#issuecomment-3737062676)) in the commit that follows (testing it in the meantime)

https://github.com/llvm/llvm-project/pull/175508

https://github.com/llvm/llvm-project/pull/175448