[clang] Avoid crash due to unimplemented StructuralValue support in the template differ

This was not implemented in https://github.com/llvm/llvm-project/pull/78041

This patch does not implement this fucntionality, 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