swift syntax should be able to recover function parameters without argument labels

[tayloraswift]

Description

SwiftParser currently can’t handle DocC-style abridged signatures such as:

init(String.Index, x:(Int) -> Int) rethrows

as they are considered malformed parse trees.

more details at https://forums.swift.org/t/swiftsyntax-how-to-repair-abbreviated-function-signatures/67602

[ahoppen]

Tracked in Apple’s issue tracker as rdar://116364935

[AppAppWorks]

I think this issue is similar to what #2727 has solved, maybe some generalization can be used to apply the solution to this issue too.