Fully qualify CLR types in migrations when needed

[roji]

Our migration and model snapshot reference property CLR types, but if the type name happens to be ambiguous, the migration doesn't compile. We need to full-qualify the CLR type with its namespace when needed.

Flagged by @kosmakoff in npgsql/efcore.pg#1976, where two enum types with the same name are referenced.

[ajcvickers]

Duplicate of #25933?

[roji]

Seems so! Though maybe we should rediscuss - we've done namespace prefixing for various other cases, and this doesn't seem like a totally exotic thing.

[ajcvickers]

Note from triage: we should do this when two types in the same model have the sample simple name. Conflicts with types outside the model is still out-of-scope.