The inverted form no longer has to replace substitutions; we use
the inverted form of DecomposeConcrete for that. So just assert
that the substitutions are equal.
A superclass requirement 'T : C' implies a layout requirement
'T : AnyObject' (if the class is @objc) or 'T : _NativeObject'
(if the class is not @objc).
In the latter case, there might already be a 'T : AnyObject'
requirement, in which case the type parameter 'T' is subject
to two layout requirements:
T : AnyObject
T : _NativeObject
The second requirement implies the first however. To encode this
in the world of rewrite loops, we the notion of a 'relation'
between property symbols, and a 'Relation' rewrite step.
Here, the relation is that _NativeObject < AnyObject. Once this
relation is recorded, the Relation rewrite step will transform
T.[layout: _NativeObject].[layout: AnyObject]
into
T.[layout: _NativeObject]
and vice versa.
This rewrite step allows us to construct a rewrite loop which
makes the first rewrite rule redundant via the second.
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.
