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

[Anutosh Bhat via cfe-commits]

anutosh491 wrote:

Let's merge, this and then have this diff (https://github.com/llvm/llvm-project/pull/175448#issuecomment-3737062676) in the commit that follows !

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