Move logic for resolving function call overloads.

This code previously lived in FunctionCallExpr but is now part
of FunctionSymbolExpr.  This change doesn't change any current
functionality, but lays groundwork for function pointers in
the language, where we'll want to do function call overload
resolution at other times besides when a function call is
actually being made.

type.h

@@ -677,7 +677,7 @@ public:
 
     const std::vector<const Type *> &GetArgumentTypes() const { return argTypes; }
     const std::vector<ConstExpr *> &GetArgumentDefaults() const { return argDefaults; }
-    const std::string &GetArgumentName(int i) const { return argNames[i]; }
+    std::string GetArgumentName(int i) const;
 
     /** @todo It would be nice to pull this information together and pass
         it when the constructor is called; it's kind of ugly to set it like