Improve Span in smir

Addressing rust-lang/project-stable-mir#31

r? ``@oli-obk``

Fix the error message for `#![feature(no_coverage)]`

When rust-lang#114656 was written, the feature flag to replace `no_coverage` was originally spelled `coverage`, but it was eventually changed to `coverage_attribute` instead.

That update happened to miss this error message in `removed.rs`, and unfortunately I only noticed just *after* the original PR was approved and merged.

cc ``@bossmc`` (original author) ``@oli-obk`` (original reviewer)
``@rustbot`` label +A-code-coverage

…kh726

Properly consider binder vars in `HasTypeFlagsVisitor`

Given a PolyTraitRef like `for<'a> Ty: Trait` (where neither `Ty` nor `Trait` mention `'a`), we do *not* return true for `.has_type_flags(TypeFlags::HAS_LATE_BOUND)`, even though binders are supposed to act as if they have late-bound vars even if they don't mention them in their bound value: 31ae3b2. This is because we use `HasTypeFlagsVisitor`, which only computes the type flags for `Ty`, `Const` and `Region` and `Predicates`, and we consequently skip any binders (and setting flags for their vars) that are not contained in one of these types.

This ends up causing a problem, because when we call `TyCtxt::erase_regions` (which both erases regions *and* anonymizes bound vars), we will skip such a PolyTraitRef, not anonymizing it, and therefore not making it structurally equal to other binders. This breaks vtable computations.

This PR computes the flags for all binders we enter in `HasTypeFlagsVisitor` if we're looking for `TypeFlags::HAS_LATE_BOUND` (or `TypeFlags::HAS_{RE,TY,CT}_LATE_BOUND`).

Fixes rust-lang#115807

… r=jackh726

Paper over an accidental regression

r? types

cc rust-lang#115781 (do not close issue until beta backport has been performed)

The PR reasons are explained with comments in the source.

In order to keep the diff simple, this PR effectively reverts rust-lang#113661, but only for RPITs. I will submit a follow up PR that fixes this correctly instead of just disabling the newly added check for RPITs. This PR should be significantly easier to review for beta backport