[clang] Re-apply "[Parse] Split incremental-extensions" (PR #66446)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> Any objections to this @vgvassilev?

I have no objections to this if that makes your workflow work. However I think this cures the symptom and not the real cause. It might be worth investigating how to adapt lldb to the incremental processing change more deeply. 

https://github.com/llvm/llvm-project/pull/66446