[cxx-interop] Fix source compatibility for NSEvent.SpecialKey.upArrow.rawValue

AppKit renames some of its C constants via API Notes, e.g. `NSUpArrowFunctionKey` is renamed into `NSEvent.SpecialKey.upArrow.rawValue`.

In this example, `NSEvent` is an existing type, but `SpecialKey` is not, so the Swift compiler synthesizes an empty type named `SpecialKey`.

The logic that was added to support import-as-member for namespaces did not account for this: it assumed that if a nested type wasn't found, then the name must be invalid.

rdar://154783494

Author: egorzhdan

lib/ClangImporter/SwiftLookupTable.cpp

@@ -359,8 +359,11 @@ SwiftLookupTable::resolveContext(StringRef unresolvedName) {
 
     // If we could not resolve this component of the qualified name, bail.
     if (!entryFound)
-      return nullptr;
+      break;
   }
+  
+  if (!parentContext)
+    return nullptr;
 
   return parentContext.getAsDeclContext()
              ? cast<clang::NamedDecl>(parentContext.getAsDeclContext())

test/Interop/Cxx/namespace/import-as-member-typechecker.swift

@@ -21,4 +21,4 @@ func takesDeepNestedStruct(_ s: MyNS.MyDeepNS.DeepNestedStruct) {
 }
 
 MyNS.method() // expected-error {{type 'MyNS' has no member 'method'}}
-MyNS.nestedMethod() // expected-error {{type 'MyNS' has no member 'nestedMethod'}}
+MyNS.nestedMethod() // expected-error {{type of expression is ambiguous without a type annotation}}

test/Interop/Cxx/objc-correctness/appkit-uikit.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend -typecheck -verify -I %S/Inputs -cxx-interoperability-mode=default %s
+// RUN: %target-swift-frontend -typecheck -verify -I %S/Inputs -cxx-interoperability-mode=default %s -swift-version 6
 
 // REQUIRES: objc_interop
 // REQUIRES: VENDOR=apple
@@ -9,6 +10,8 @@ import AppKit
 
 var _: AttributeScopes.AppKitAttributes.UnderlineStyleAttribute! = nil
 
+var _ = NSEvent.SpecialKey.upArrow.rawValue
+
 #elseif canImport(UIKit)
 import UIKit