Function types didn't always degrade to pointers like they should.  Introduced and used Csyntax.typeconv to address this issue and reduce the number of cases in the classify functions

git-svn-id: https://yquem.inria.fr/compcert/svn/compcert/trunk@1266 fca1b0fc-160b-0410-b1d3-a4f43f01ea2e