[PATCH] Adjust lazy macro definition

[Nathan Sidwell]

We provide a lazy macro definition hook, to deal with the FP macros 
where it is expensive to generate the string representation of the FP 
value.  The current hook is much more general than needed, but it's not 
clear that generality can be used because we don't document what macro 
state can be manipulated.  We create the macro early with a bogus FP 
token, then overwrite the identifier node making it look like a builtin 
node with an out-of-range index.  Upon first use we call the hook which 
checks the index value, and if ok updates with the saved macro after 
nadgering the token of interest with the correct string.

This patch makes things cleaner.  We don't overload the builtin index 
meaning, rather we add a lazy field to cpp_macro, and allow the user to 
control that.  There's plenty of spare space there, so this doesn't 
increase the macro size.

Rather than have the user's hook fish out the cpp_macro object, we 
provide the object directly, along with the lazy hook value for it to 
inspect.  It can then update the macro as desired and declare victory.

We already have _cpp_maybe_notify_macro_use that deals with lazy 
definition, it's simply a matter of adjusting that and using it in a few 
more place that are now easier.

Finally, the PCH machinery was invoking the lazy hook during PCH write 
out.  This seems fragile to me -- changing the memory state during 
serialization.  We don't need to do that, as the lazy machinery is all 
GTY marked.  This does mean we'll lazily define the PCH'd macro upon 
every first use after reading back, but that doesn't seem costly to me.

booted & tested on x86_64-linux.

nathan
-- 
Nathan Sidwell
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