Pre-generate names ahead of time

[rbuckton]

This change ensures that we pre-generate all names in a lexical scope before descending the tree. This allows us to ensure that generated names that must be preserved in nested scopes are allocated at the right time before traversing the tree. In addition to allowing us to remove the SkipNameGenerationScope flag for generated identifiers, this sets up some other future PRs that will need name generation to occur at an earlier time.

For most of our performance suite the performance cost is not statistically significant. However, there is a very small increase in emit time (over one second) for very large projects (though still not statistically significant compared to an average emit time of over one minute).

[weswigham]

I like the side effect that the var declarations are in order now (even if the initial uses can now be out-of-order).