[ClangImporter] Hardcode NSString being bridged to String (#17412) (#17430)

...so that we don't have to depend on the ObjectiveC module having API
notes for a class it doesn't actually define.

Support for rdar://problem/40278479.

(cherry picked from commit 426d27c)

Author: jrose-apple

lib/ClangImporter/ImportType.cpp

@@ -949,12 +949,13 @@ namespace {
         }
 
         // Determine whether this Objective-C class type is bridged to
-        // a Swift type.
+        // a Swift type. Hardcode "NSString" since it's referenced from
+        // the ObjectiveC module (a level below Foundation).
         Type bridgedType;
         if (auto objcClassDef = objcClass->getDefinition())
           bridgedType = mapSwiftBridgeAttr(objcClassDef);
-        else
-          bridgedType = mapSwiftBridgeAttr(objcClass);
+        else if (objcClass->getName() == "NSString")
+          bridgedType = Impl.SwiftContext.getStringDecl()->getDeclaredType();
 
         if (bridgedType) {
           // Gather the type arguments.