[clang] [Parse] Split incremental-extensions (PR #65683)

[Ben Barham via cfe-commits]

bnbarham wrote:

> LGTM! Maybe we can cherry pick it for the patch release 17.0.1. If that’s a critical patch for swift we can try for the 17.0.0 release.

It's in the appropriate branches in our fork, so it's fine for us either way. I suggested it more because of https://github.com/llvm/llvm-project/issues/62413 than anything else.

https://github.com/llvm/llvm-project/pull/65683