Rollup merge of #145378 - xizheyin:144968, r=davidtwco

Add `FnContext` in parser for diagnostic

Fixes rust-lang/rust#144968

Inspired by rust-lang/rust#144968 (comment), 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 And I removed the outdated note.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

Author: Zalathar