sourcekitd: define a new custom buffer for expression type response.

This custom buffer encapsulates the memory layout details of the response
for the expression type request. From the client side, each expression type
is represented as a tuple of {expr_offset, expr_length, printed_type}.

rdar://35199889

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -2057,8 +2057,7 @@ void SwiftLangSupport::collectExpressionTypes(StringRef FileName,
       llvm::raw_svector_ostream OS(TypeBuffer);
       ExpressionTypesInFile Result;
       for (auto Item: collectExpressionType(*SF, Scratch, OS)) {
-        Result.Results.push_back({Item.offset, Item.length, Item.typeOffset,
-          Item.typeLength});
+        Result.Results.push_back({Item.offset, Item.length, Item.typeOffset});
       }
       Result.TypeBuffer = OS.str();
       Receiver(Result);