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

[Ben Barham via cfe-commits]

bnbarham wrote:

> 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.

To be clear, this is re-applying https://github.com/llvm/llvm-project/pull/65683 such that it is actually the NFC it was intended to be. I've closed the LLDB PR (https://github.com/llvm/llvm-project/pull/66271), so it's still using `-fincremental-extensions`.

The actual use case for splitting these is eg. Swift's clang importer, where the need is very similar to https://github.com/llvm/llvm-project/issues/62413 and IMO it makes sense to keep these two as separate, with `-fincremental-extensions` implying incremental processing.

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