In Swift 6 and later, whitespace is no longer permitted between an
attribute name and the opening parenthesis. Update the parser so that
in Swift 6+, a `(` without preceding whitespace is always treated as
the start of the argument list, while a '(' with preceding whitespace is
considered the start of the argument list _only when_ it is unambiguous.
This change makes the following closure be parsed correctly:
{ @MainActor (arg: Int) in ... }
rdar://147785544
