[Clang][Attributes] Add MeaningfulToClassTemplateDefinition to unavailable attribute

There may be cases in which we want to diagnose a type as unavailable but it may
not be complete at the time. Setting MeaningfulToClassTemplateDefinition fixes
this issue.

This fixes: https://github.com/llvm/llvm-project/issues/61815

Differential Revision: https://reviews.llvm.org/D147495