Patch: stab info for const fields

[Richard Kenner]

    Again, this goes to Zack's point; we can allow two types to be compatible,
    even if they are not the same.  (That idea already exists in C/C++)

So the idea is that we allow "compatible" types on the two operands and
the result of a PLUS_EXPR, but they are not required to all be the same?
We've never really been clear on that and there are three possibilities:

(1) The types are exactly the same.
(2) TYPE_MAIN_VARIANT of the types are the same.
(3) The types are "compatible" (to be defined later).

I've argued for (1) in the past and continue to do so.  Is the above
arguing for (2) or (3)?