cpplib: Start moving switch handling to front ends

[Stan Shebs]

Zack Weinberg wrote:

>"Darwin land" needs to stop using #import.
>
>No, really.  The semantics of #import (and #pragma once) require that
>the compiler be able to determine whether two include requests refer
>to the same file, with 100% accuracy, for _correctness_.  This is
>impossible without making unportable assumptions about file system
>semantics, which cpplib does not do.
>
Heh, we tried that argument with our users, but with literally thousands
of headers assuming #import or #pragma once, both internal and external
to Apple, plus an Objective-C book or two recommending the practice,
the response was, shall we say, "negative".  It was pretty clear they
were going to keep firing compiler folks until they found one who was
willing to un-deprecate #import. 1/2 :-)

In the users' defense, it must be said that #import has been working
well on a huge amount of code - in the 2 1/2 years that I've been at
Apple, we've seen dozens of ICEs for instance, but no failures of
#import to find the right file.  So it's not surprising that users
wonder why we're trying to fix something that doesn't seem broken.

Stan