This takes some trickery since CastExpr has subclasses (and indeed,
is abstract).

Also, smoosh the CastKind into the bitfield from Expr.

Drops two words of storage from Expr in the common case of expressions
which don't need inheritance paths.  Avoids a separate allocation and
another word of overhead in cases needing inheritance paths.  Also has
the advantage of not leaking memory, since destructors for AST nodes are
never run.

llvm-svn: 110507