When `MemberImportVisibility` is enabled and a declaration from a cross import
overlay is diagnosed because it has not been imported, suggest imports of the
declaring and bystanding modules instead of the cross import overlay module
(which is an implementation detail).
Resolves rdar://149307959.
If a clang module declares a cross-import overlay, but it also has a traditional overlay, we want the cross-import overlay to be registered with the SourceFile as sitting atop the traditional overlay. Otherwise module-qualified name lookups will bypass the cross-import overlay.
Fixes rdar://62139656.
These are mostly harmless, except that they make the two module names synonymous in qualified lookup. A hard error seems too aggressive for something that could easily be caused by uncoordinated changes to two modules, so warn instead.
