Sourcekit/DocSupport: fix an assertion when generating documentation for extensions with attributes

When sanitizing the documentation comments for synthesized extensions,
we expect some text like "<declaration>extension". This isn't the case
when use-facing attributes are present.

rdar://50913510

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -368,7 +368,7 @@ static bool initDocEntityInfo(const Decl *D,
       StringRef DocRef = (StringRef)DocBuffer;
       if (IsSynthesizedExtension &&
           DocRef.find("<Declaration>") != StringRef::npos) {
-        StringRef Open = "<Declaration>extension ";
+        StringRef Open = "extension ";
         assert(DocRef.find(Open) != StringRef::npos);
         auto FirstPart = DocRef.substr(0, DocRef.find(Open) + (Open).size());
         auto SecondPart = DocRef.substr(FirstPart.size());