Get rid of empty IrBase class.

It isn't useful in any way now that the GC is out of the
picture, and the existance of a "dummy" toChars() method is
annoying when debugging.

ir/irfunction.h

@@ -89,7 +89,7 @@ private:
 };
 
 // represents a function
-struct IrFunction : IrBase
+struct IrFunction
 {
     // constructor
     IrFunction(FuncDeclaration* fd);