Remove lowering for inline const patterns and cleanup parsing

[nbdd0121]

Follow up to rust-lang/rust#138492 to remove the lowering of inline const that still remains.

Only visible change is that when reporting the error, the "const" keyword is covered in span too, which I think is better.

r? @lcnr

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred in match checking

cc @Nadrieril

[Nadrieril]

cc @dianne

[nbdd0121]

I'd prefer to land this via #149667. Otherwise, this looks good to me, apart from a couple nits.

Thanks for mentioning, I wasn't aware of that PR. I'll close this one then. If that one stalls please let me know and I'll re-open.