Remove `Symbol` from `Named` variant of `BoundRegionKind`/`LateParamRegionKind`
The `Symbol` is redundant, since we already store a `DefId` in the region variant. Instead, load the name via `item_name` when needed (which is almost always on the diagnostic path).
This introduces a `BoundRegionKind::NamedAnon` which is used for giving anonymous bound regions names, but which should only be used during pretty printing and error reporting.
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