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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

@mcbarton, @anutosh491 can we check if the wasm changes compile and work as expected?

cc: @aaronj0.

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