Simplify Inheritable.computeCanonicalEnclosingContainer.

[srawlins]

This method is very complicated, so any simplification is handy.

• It used to start with a very long if-then body, and the else body was rather lost far below. Instead we can implement some simple short circuiting, if we're not dealing with an inherited element.
• It used to carry around a found variable, breaking the loop when it found a container. But it's simpler to just return at that point.
• The final return value is always null, representing a state where no canonical enclosing container is found. Just return null to simplify understandability.

Otherwise this is a no-op.