Rollup merge of #114662 - fmease:lazy-ty-aliases-unlock-trailing-wcs, r=oli-obk

Unlock trailing where-clauses for lazy type aliases

Allows trailing where-clauses on lazy type aliases and forbids[^1] leading ones.
Completes #89122 (see section *Top-level type aliases*).

`@rustbot` label F-lazy_type_alias
r? `@oli-obk`

[^1]: This is absolutely fine since lazy type aliases are only meant to be stabilized as part of a new edition.

Author: compiler-errors