[CodeCompletion] Add a field NotRecommendedReason to code completion result to indicate why an item is not recommended, NFC. rdar://25415947

As implied in rdar://24818863, striking through a module name may be an overkill to suggest the module is redundant to import. We try to
fine-grain not-recommended-reason so that proper UI cue can be adopted in the future.

tools/SourceKit/lib/SwiftLang/CodeCompletionOrganizer.cpp

@@ -1159,6 +1159,7 @@ Completion *CompletionBuilder::finish() {
       base = SwiftResult(semanticContext, current.getNumBytesToErase(),
                          completionString, current.getAssociatedDeclKind(),
                          current.getModuleName(), current.isNotRecommended(),
+                         current.getNotRecommendedReason(),
                          current.getBriefDocComment(),
                          current.getAssociatedUSRs(),
                          current.getDeclKeywords());