Sema: Improve MemberImportVisibility diagnostics for cross import overlays.

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.

test/CrossImport/Inputs/lib-templates/lib/swift/DeclaringLibrary.swiftinterface

@@ -3,5 +3,7 @@
 
 import Swift
 
-public struct DeclaringLibraryTy {}
+public struct DeclaringLibraryTy {
+  public init()
+}
 public struct ShadowTy {}