[Serialization] Serialize subclassScope to keep linkage

subclassScope was always set as NotApplicable when deserialized but we
need to serialize and deserialize it to keep correct linkage when using
SIB

```swift
open class Visitor {
    public func visit() {
        visitExprImpl()
    }
    @_optimize(none)
    private func visitExprImpl() {
    }
}
```

In this case, `visitExprImpl` is private but subclassScope is External.
So it should be lowered as an external function at LLVM IR level.

But once it's serialized into SIB, subclassScope of `visitExprImpl` was
deserialized as NotApplicable because it was not serialized. This
mismatch makes `visitExprImpl` lowered as an internal function at LLVM
IR level.

So `subclassScope` should be serialized.

lib/Serialization/ModuleFormat.h

@@ -55,7 +55,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 568; // removed UTF16
+const uint16_t SWIFTMODULE_VERSION_MINOR = 569; // subclass scope
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///