swift234

[Clang importer] Clean up creation and conflict checking of CF "Ref" type aliases.

CF types are generally imported twice, once with the "Ref" and once
without the "Ref", unless the non-"Ref" version would conflict with
some other declaration (e.g., an Objective-C class of the same name).

The logic to create both versions was a bit convoluted, and tended to
return the "Ref" version as the primary declaration, which conflicts
with our general inclination to use the non-"Ref" version. Now we create
the non-"Ref" version directly and then create the aliased "Ref" version
as an alternate declaration. The "Ref" version is a typealias of the
non-"Ref" version, and the "Ref" version is never used anywhere else
in the AST.

Also, improve the conflict resolution for cases where the only
conflicting declaration is an opaque tag used to declare the CF type
itself, e.g.,

  typedef struct CGImage *CGImageRef;

These cases used to be considered conflicts, but they are benign
because the underlying struct will never actually be imported. Make
them not conflicts, so we get the preferred name "CGImage" rather than
"CGImageRef".