minor: Remove impossible function pointer qualifiers from grammar

[Giga-Bowser]

Rust doesn't support const or async function pointers, nor does our parser.

[lnicola]

We might want to parse these so we can offer diagnostics like rustc: "error: an fn pointer type cannot be async".

[Giga-Bowser]

That diagnostic is offered as it stands, with an accompanying assist, although with a lot of cruft afterwards. We could probably just eat the tokens to fix this. Is there a reason to emit a proper error from the parser rather than relying on the diagnostic from rustc?

[lnicola]

I like to disable check and rely on the native r-a diagnostics. They're (arguably) faster, and get displayed on-the-fly, not just when saving the file.

Ideally, we would be able to reuse the rustc diagnostic. Not going to happen, but we do have a couple of them implemented.

[Giga-Bowser]

Whoops, of course I pushed that right before seeing your reply. I'll eventually make a new PR to generate a proper rust-analyzer diagnostic for this. For now, I'll just close this and handle this stuff there.