[clang] Avoid crash due to unimplemented StructuralValue support in the template differ (#93265)

This was not implemented in
https://github.com/llvm/llvm-project/pull/78041 when StructuralValue
TemplateArguments were originally added.

This patch does not implement this functionality, it just falls back to
the expression when possible.

Otherwise, such as when dealing with canonical types to begin with, this
will just ignore the argument as if it wasn't even there.

Fixes https://github.com/llvm/llvm-project/issues/93068