gengtype needs for C++?

[Tom Tromey]

Ian> In Tom's interesting idea, we would write the mark function by hand for
Ian> each C++ type that we use GTY with.

I think we should be clear that the need to write a mark function for a
new type is a drawback of this approach.  Perhaps gengtype could still
write the functions for ordinary types in GCC, just not (templatized)
containers.

Also, perhaps we actually need 2 such functions, one for the GC and one
for PCH.  I don't remember.

Tom