AST: Decl::isPotentiallyOverridable() returns false for protocol requirements

The way this predicate is used is closely intertwined with 'can have
open access'. For example, we don't want protocol requirements coming
from testable imports to have 'open' access level.

Author: slavapestov

include/swift/AST/Decl.h

@@ -7177,8 +7177,7 @@ inline bool Decl::isPotentiallyOverridable() const {
       isa<SubscriptDecl>(this) ||
       isa<FuncDecl>(this) ||
       isa<DestructorDecl>(this)) {
-    return getDeclContext()->getSelfClassDecl() ||
-           isa<ProtocolDecl>(getDeclContext());
+    return getDeclContext()->getSelfClassDecl();
   } else {
     return false;
   }