Make the switch over Keywords in TokenPrecedence.swift exhaustive

[ahoppen]

Fix this To Do by making the switch exhaustive. We should consider all the keywords that we add cases for and decide which token precedence they should get.

https://github.com/apple/swift-syntax/blob/2e3c42cf38defd998c87ecfe8df138f925d22736/Sources/SwiftParser/TokenPrecedence.swift#L236

[ahoppen]

Tracked in Apple’s issue tracker as rdar://113929347