cpplib: Start moving switch handling to front ends

[Neil Booth]

Zack Weinberg wrote:-

> |    In the present implementation, a single use of `#import' will
> | prevent the file from ever being read again, by either `#import' or
> | `#include'.  You should not rely on this; do not use both `#import'
> | and `#include' to refer to the same header file.
> 
> Is #pragma once in active use as well as #import?  Neil wanted to
> ditch #pragma once.

Hey! Not just me -- you're guilty as charged, too!

Neil.