one-time diagnostic and suggestion for reëxporting private variant error

We issue just one message for an erroneous glob private variant reëxport
(using the Session's one-time-diagnostics capability), but individual
(non-glob) such erroneous reëxports still get their own messages. The
suggestion to make the enum public is also one-time.

The enum variant reëxport error didn't have an associated error code
(and remedying this here is deemed out of the scope of this commit), so
we resort to the expediency of using 0 as the `DiagnosticMessageId`
value.

Adding Debug to NameResolution was helpful in development.

This resolves #46209.

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