[SourceKit] Include generated macro buffers in diagnostic responses

Introduce a new key `generated_buffers`, which
stores an array of generated buffers. These
include the buffer text, as well as its original
location and any parent buffers.

While here, also fix rdar://107281079 such that
only apply the filename fallback logic to the
pretty-printed Decl case. We ought to remove this
fallback once the editor can handle it though.

rdar://107281079
rdar://107952288

unittests/SourceKit/SwiftLang/CursorInfoTest.cpp

@@ -86,9 +86,8 @@ class NullEditorConsumer : public EditorConsumer {
 
   bool diagnosticsEnabled() override { return false; }
 
-  void setDiagnosticStage(UIdent DiagStage) override {}
-  void handleDiagnostic(const DiagnosticEntryInfo &Info,
-                        UIdent DiagStage) override {}
+  void handleDiagnostics(ArrayRef<DiagnosticEntryInfo> DiagInfos,
+                         UIdent DiagStage) override {}
   void recordFormattedText(StringRef Text) override {}
 
   void handleSourceText(StringRef Text) override {}