Teach the parser to recognize a top-level '->' in a closure as a signature and
produce a targeted diagnostic ("expected 'in' after the closure signature")
instead of treating it as a type expression.
Adds a parser test:
- test/Parse/closure-missing-in.swift
Updates existing test with the new diagnostic:
- test/Parse/type_expr.swift
Resolves: swiftlang/swift#59928
