Note ["Arising from" messages in generated code]

Consider code generated when we desugar code before typechecking;
see Note [Rebindable syntax and XXExprGhcRn].

In this code, constraints may be generated, but we don't want to
say "arising from a call of foo" if 'foo' doesn't appear in the
users code.  We leave the actual CtOrigin untouched (partly because
it is generated in many, many places), but suppress the "Arising from"
message for constraints that originate in generated code.