Rollup merge of #120704 - amandasystems:silly-region-name-rewrite, r=compiler-errors

A drive-by rewrite of `give_region_a_name()`

This drive-by rewrite makes the cache-updating nature of the method clearer, using the Entry API into the hash table for region names to capture the update-insert nature of the method. May be marginally more efficient since it only runtime-borrows and indexes the map once, but in this context the performance impact is almost certainly completely negligible.

Note that this commit should preserve all externally visible behaviour. Notably, it preserves the debug logging:

1. printing even in the case of a `None` for the new computed name, and
2. only printing on new values, begin silent on reused values

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