[RFC] type safe trees

[Richard Kenner]

    Clearly it's not what is happening, as people are complaining about
    the effect of your changes on data structure size for the C++ frontend.

But my changes aren't Ada-specific!  They are needed to support
constructs that have always been supported in GCC, but weren't yet
implemented in tree-ssa.  Ada uses them *more* than other languages,
but it's not Ada-specific, nor is it really front-end related.