Swaps out all uses of CapturedVariablesByLambda for functions on the
Scope tree.

The reason the baseline changed is that the order of enumerating closures changed. I briefly looked into matching the behavior, but the problem is that the closure order in the old visitors is determined by the visitation of captured variables, not visitation of the closures themselves. More specifically, an item is only added to the CapturedVariablesByLambda dictionary when ReferenceVariable encounters a captured variable.

In contrast, the visitation of the Scope tree looks at closures in an in-order traversal of the tree, stopping at the first introduced closure, not necessarily the closure which first captured a variable.

One obvious consequence of this changing is that the old ordering for a series of nested lambdas was effectively post-order -- the interior closures would be added to the visitation first, since they would be added from the perspective of the captured variable looking up, instead of the visitor looking down. This particular case is responsible for all the baseline changes that I have seen.

When I looked into replicating this traversal order I found it complicated and pretty counter-intuitive. Since the baseline change was so low in emitted IL, I thought it better to keep a very natural in-order tree traversal and just rebaseline a few tests.

Fixes https://github.com/dotnet/roslyn/projects/26#card-3753318