Typedtree: add type information to primitive coercions

This will enable type-directed optimizations during translation of
module coercions.

WARNING: this breaks Typedtree representation, magic number will have to
be adjusted.

From: Frederic Bour <frederic.bour@lakaban.net>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15582 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02