[Sema] Diagnose exportability of decls limited by a non-public import

Check the API of decls to ensure that imported types are only used in
decls with compatible access-level. For example, error on using a type
import as internal in a public function, but allow it in internal
functions and lower.

This change raises errors on wrongful use of types limited by an import
and offers a note pointing to the import that did limit the imported
type.

Author: xymus

include/swift/AST/DiagnosticsSema.def

@@ -2064,6 +2064,9 @@ ERROR(access_level_on_import_unsupported, none,
 ERROR(access_level_conflict_with_exported,none,
       "'%0' is incompatible with %1; it can only be applied to public imports",
       (DeclAttribute, DeclAttribute))
+NOTE(module_imported_here,none,
+     "module %0 imported as '%select{private|fileprivate|internal|package|%ERROR|%ERROR}1' here",
+     (Identifier, AccessLevel))
 
 // Opaque return types
 ERROR(opaque_type_invalid_constraint,none,