[clang] [clang] Remove dead incremental Parser code (PR #102450)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> > As far as I can tell, -fincremental-extensions should set the language option IncrementalExtensions which in turn is the default for Preprocessor::IncrementalProcessing.
> 
> It is true that it is the default, but it's possible for clients to have one without the other. This is the case in Swift in particular (see #65683).
> 
> > The few tests we have with -fincremental-extensions pass with this change.
> 
> I couldn't think of a great way to test this at the time. If anyone has any ideas though... 😅. Otherwise we could add a comment here to explain why it's needed?

If I understand correctly, you are manually calling `Preprocessor::enableIncrementalProcessing`? This should be added as a unittest because right now it is not exercised at all...

https://github.com/llvm/llvm-project/pull/102450