[DebugInfo][Verifier] Verify that array types have base types present (#70803)

The base-type field of a DICompositeType is optional in the IR syntax,
however it makes no sense to have an array of an unspecified type. Such
debug-info would be meaningless, and the added test crashes otherwise
(see #70787). Thus, add a verifier check to reject such ill-formed debug
info metadata. Test produced by Christian Ulmann.

fixes #70787