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

[Ben Barham via cfe-commits]

bnbarham wrote:

Seems Swift wasn't the only project to run into this, eg. https://github.com/llvm/llvm-project/issues/62413#issuecomment-1526499543.

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