This is virtually certain to cause regressions, needs crater.

In #50781 it was discovered that our object safety rules are not sound because we allow `Self` in where clauses without restrain. This PR is a direct fix to the rules so that we disallow methods with unsound where clauses.

This currently uses hard error to measure impact, but we will want to downgrade it to a future compat error.

Fixes #50781.

r? @nikomatsakis

This commit moves the calculation of the `LanguageItems` structure into a
query rather than being calculated before the `TyCtxt` exists, with the eventual
end goal of removing some `CrateStore` methods.

Addresses the second part of #42171 by removing the `TraitRef` from
`ProjectionTy`, and directly storing its `Substs`.

Closes #42171.

Instead, thread around `Option<CodeExtent>` where applicable.

Fixes #38604

needs a crater run

This should be the last time I fix this function.

Fixes #38404.

The warnings have already reached stable

The test rfc1592_deprecated is covered by `bad_sized` and
`unsized6`.

Fixes #33242
Fixes #33243