Perform synthesis of closure methods in an early pass before visitation,
meaning we no longer need to do a second visitation of the tree to lower
local functions.

The baselines have been changed because we now do closure id and
synthessis in order of closure visitation, rather than bound node
visitation. Closure visitation visits all the closures in a given scope,
then recurs into nested scopes, while BoundNode visitation treats
closures as scopes themselves, so nested closures are visited before
closures declared in the same scope.

Fixes https://github.com/dotnet/roslyn/projects/26#card-3753331