Pitch: Improved ClangImporter Diagnostics

[benrimmington]

@allevato

I've always wondered if it would be possible to import incomplete types like this in Swift as caseless enums (or a memberless struct with a private initializer) instead, with the actual type name, and have pointers to them just be Unsafe(Mutable)Pointer<X>. It seems like that would help to make opaquifying C structs still type-preserving in Swift.

OpaquePointer was previously discussed (circa Swift 4.2):

C Interoperability: Import "struct Incomplete *" as Unsafe(Mutable)RawPointer rather than OpaquePointer

Hi all, TL;DR I propose to replace the OpaquePointer struct with a deprecated typealias for UnsafeRawPointer. Then, change the import of C pointers-to-incomplete-types to produce UnsafeMutableRawPointer or UnsafeRawPointer, depending on whether the pointee const. Introduction The standard library currently has an odd pointer type, OpaquePointer. Better abstractions exist in the standard library for working with pointers to "raw" memory (Unsafe(Mutable)RawPointer), so OpaquePointer is mostly …