[Pitch] Allow conditional compilation in comma-separated syntactic lists

Introduction Swift's conditional compilation directives currently work well around declarations and statements, but they cannot be used to conditionally include individual elements inside many comma-separated syntactic lists. This pitch proposes allowing #if / #elseif / #else / #endif blocks at element boundaries inside selected list grammars, beginning with function and initializer parameter lists. The goal is not to introduce C-style textual preprocessing. Conditional compilation would remai...