Add a diagnostic attribute for special casing const bound errors for non-const impls
Somewhat of a follow-up to https://github.com/rust-lang/rust/pull/144194
My plan is to resolve
f4e19c6878/compiler/rustc_hir_typeck/src/callee.rs (L907-913)
but doing so without being able to mark impls the way I do in this PR wrould cause all nice diagnostics about for loops and pointer comparisons to just be a `*const u32 does not implement [const] PartialEq` errors.