Clean up a few general patterns that are now obviated by canImport
This aligns more generally with the cleanup that the Swift Package
Manager has already done in their automated XCTest-plumbing tool in
apple/swift-package-manager#1826.
In theory there could be a "fixed-layout" enum that's not exhaustive
but promises not to add any more cases with payloads, but we don't
need that distinction today.
(Note that @objc enums are still "fixed-layout" in the actual sense of
"having a compile-time known layout". There's just no special way to
spell that.)
- Add newlines where missing to match sys/errno.h error groups separation
- Prefer single-line comments to C-style comments
- Align most comments to same column
- Add comment following #endif at EOF to show what it ends
- Remove extraneous whitespace
