[CPP PATCH] node type

[Nathan Sidwell]

Finally, this gets to clean up the convoluted way the preprocessor marks 
the type of identifier nodes.  Right now it's a combination of a type 
field and a pair of flags, which gives rise to some convoluted tests and 
remapping for GTY.  This patch rationalizes it by using the type field 
exclusively.

Rather than a single NT_MACRO type, we now have NT_USER_MACRO and 
NT_BUILTIN_MACRO types.  Checking for either of them is a simple bit 
mask, as there's a unique set bit they share.  NT_MACRO_MASK is that bit.

Another change is that NT_ASSERTION is not needed.  Assertions are in a 
disjoint namespace (by prepending '#'), and we can simply use the 
non-nullness of the answers field to know there's an assert (possibly 
checking for NT_VOID if we're looking at an arbitrary node).  We have to 
set the answer field to NULL when clearing a node -- before we could 
leave a dangling pointer.

With NT_MACRO_ARG replacing NODE_MACRO_ARG we have exactly 4 NT_$FOO 
values (which is what we had before), but we free up 2 NODE_ flags, 
which is nice.  The NT_ values directly map to the GTY machinery, so we 
don't need a translation layer anymore.

booted and tested on x86_64-linux.  applying to trunk.

nathan
-- 
Nathan Sidwell
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