DebugInfo: Stop modifying Operation::Error inside of verify()

The only caller of Operation::verify() is DWARFExpression::verify(),
which iterates past the (ephemeral) Operation immediately after.

- Stop setting Operation::Error because the mutation will never be
  observed.
- Change verify() to a static function to be sure all callers are
  updated.

Differential Revision: https://reviews.llvm.org/D113957