swift234

rework the canonical type system a bit to make it so that all Types can access
their ASTContext without bloating everything.  Basically, now instead of having
a canonical type point to itself when canonical, it points to the ASTContext 
when canonical.  This means that Canonical types always have direct access to
their context and non-canonical ones just indirect through the canonical type
pointer to get to it (so it's two jumps away) by using a PointerUnion.

This should make type manipulation more straight-forward.

While we're at it, we actually memoize type canonicalization.  Before we 
forgot to remember the result in the canonical type pointer in Type.


Swift SVN r755