Patch for constant expression constraints in preprocessor

[Neil Booth]

Joseph S. Myers wrote:-

> Although constant expression constraints in the compiler are extremely
> complicated, those in the preprocessor are simple (largely because of
> the lack of casts, sizeof and floating point constants).  (The
> _semantics_ are also simple once the correction of DR#265 is applied.)

I don't believe we follow 265 (and personally have no intention of
doing so - it seems daft not to follow target arithmetic).  Nor
does EDG from what I can tell.

Unless I misunderstand.

Neil.