Add support for function pointers.

Both uniform and varying function pointers are supported; when a function
is called through a varying function pointer, each unique function pointer
value across the running program instances is called once for the set of
active program instances that want to call it.

expr.h

@@ -490,6 +490,7 @@ public:
     Expr *TypeCheck();
     Expr *Optimize();
     int EstimateCost() const;
+    llvm::Constant *GetConstant(const Type *type) const;
 
     const Type *type;
     Expr *expr;
@@ -568,6 +569,7 @@ public:
     Expr *Optimize();
     void Print() const;
     int EstimateCost() const;
+    llvm::Constant *GetConstant(const Type *type) const;
 
     bool ResolveOverloads(const std::vector<const Type *> &argTypes);
     Symbol *GetMatchingFunction();
@@ -586,6 +588,8 @@ private:
 
     /** The actual matching function found after overload resolution. */
     Symbol *matchingFunc;
+
+    bool triedToResolve;
 };
 
 
@@ -604,6 +608,20 @@ public:
 };
 
 
+/** @brief An expression that represents a NULL pointer. */
+class NullPointerExpr : public Expr {
+public:
+    NullPointerExpr(SourcePos p) : Expr(p) { }
+    
+    llvm::Value *GetValue(FunctionEmitContext *ctx) const;
+    const Type *GetType() const;
+    Expr *TypeCheck();
+    Expr *Optimize();
+    void Print() const;
+    int EstimateCost() const;
+};
+
+
 /** This function indicates whether it's legal to convert from fromType to
     toType.
  */