When a rewrite rule is replaced with a path containing ::Adjust, ::Decompose,
::ConcreteConformance or ::SuperclassConformance rewrite steps, the steps
will get a non-zero EndOffset if the original rule appears in a step with a
non-zero EndOffset.
For this reason, these steps must work with a non-zero EndOffset, which
primarily means computing correct offsets into the term being manipulated.
Using StartOffset to encode the prefix length is wrong, because
then we get an invalid rewrite step after replaceRuleWithPath().
Instead, encode the prefix length in the RuleID field.
