re-re-land of skslc now automatically turns on derivatives support

src/sksl/ir/SkSLFunctionDeclaration.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
  
 #ifndef SKSL_FUNCTIONDECLARATION
 #define SKSL_FUNCTIONDECLARATION
 
 #include "SkSLModifiers.h"
 #include "SkSLSymbol.h"
 #include "SkSLSymbolTable.h"
 #include "SkSLType.h"
 #include "SkSLVariable.h"
 
 namespace SkSL {
 
 /**
  * A function declaration (not a definition -- does not contain a body).
  */
 struct FunctionDeclaration : public Symbol {
     FunctionDeclaration(Position position, std::string name, 
                         std::vector<const Variable*> parameters, const Type& returnType)
     : INHERITED(position, kFunctionDeclaration_Kind, std::move(name))
     , fDefined(false)
+    , fBuiltin(false)
     , fParameters(std::move(parameters))
     , fReturnType(returnType) {}
 
     std::string description() const override {
         std::string result = fReturnType.description() + " " + fName + "(";
         std::string separator = "";
         for (auto p : fParameters) {
             result += separator;
             separator = ", ";
             result += p->description();
@@ skipping 11 matching lines @@
         }
         for (size_t i = 0; i < fParameters.size(); i++) {
             if (fParameters[i]->fType != f.fParameters[i]->fType) {
                 return false;
             }
         }
         return true;
     }
 
     mutable bool fDefined;
+    bool fBuiltin;
     const std::vector<const Variable*> fParameters;
     const Type& fReturnType;
 
     typedef Symbol INHERITED;
 };
 
 } // namespace
 
 #endif