[MiscDiagnostics] Modernize `checkUseOfMetaTypeName`
This is a follow-up to https://github.com/swiftlang/swift/pull/86499.
Make sure that the error is downgraded even when there is no parent
expression, this covers pattern binding initializer expressions which
started to error after `DynamicSelf` change.
Replace a custom "will be an error in Swift 6" with a modern
downgrade API.
