Add `FnContext` in parser for diagnostic
Fixesrust-lang/rust#144968
Inspired by https://github.com/rust-lang/rust/issues/144968#issuecomment-3156094581, I implemented `FnContext` to indicate whether a function should have a self parameter, for example, whether the function is a trait method, whether it is in an impl block. And I removed the outdated note.
I made two commits to show the difference.
cc ``@estebank`` ``@djc``
r? compiler
dc047f1385