[CodeCompletion] Add 'IsSystem' flag to code completion result item

'key.is_system: 1' is added if the associated declaration is from a
system module.

rdar://problem/62617558

tools/SourceKit/lib/SwiftLang/SwiftCompletion.cpp

@@ -544,6 +544,7 @@ bool SwiftToSourceKitCompletionAdapter::handleResult(
   Info.ModuleName = Result->getModuleName();
   Info.DocBrief = Result->getBriefDocComment();
   Info.NotRecommended = Result->isNotRecommended();
+  Info.IsSystem = Result->isSystem();
 
   Info.NumBytesToErase = Result->getNumBytesToErase();