Add annotations to the graphviz region graph on region origins

This adds
- (ex) for regions whose origin is existential,
- (p) for regoins whose origin is a placeholder, and
- (p for <name>) if the originating placeholder is named.

This has helped _my_ debugging and it doesn't create too bad clutter, I feel.

The change is ridiculously small, but I turned it into a separate PR so we can bikeshed the format.

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