[llvm-tblgen] Support conditional definitions using !casts clauses

This is a follow on to D145108. This started as simply fixing the crash on an error case reported against that change, but I think this also ends up fixing the original reported issue (https://github.com/llvm/llvm-project/issues/49830) as well. More accurately, D145108 fixed the case where the cast resolves to an existing record, and this change fixes the case where the named record doesn't exist.

Differential Revision: https://reviews.llvm.org/D145711