[Proof of concept] Support the use of optional labels for the first trailing closure

[xwu]

Most of the changes are minor refactorings that could benefit the codebase regardless of the feature in question being implemented (such as typo corrections, minor renamings for clarity); I will extract those out for actual review.

Missing are any diagnostics improvements, IDE integration, or even tests. This PR is not intended for merging as-is. However, it does build, and the resultant compiler will accept the following:

if let x = [1, 2, 3].first where: { $0 > 2 } {
  print(x)
}

[DevAndArtist]

Irrelevant to the actual feature, but the tooling that will eventually compute the syntax highlighting would need to be taught to not interpret where: as the where clause. Would that need to be part of this feature still?

Great work!