Firstorder: record with defined field aren't conjonctions (fix #2629)

 The let-in in the type of constructor was perturbating the
 Hipattern.dependent check, leading firstorder to consider too much
 inductives as dependent-free conjonctions, ending with lift errors
 (UNBOUND_REL_-2, ouch).

 This patch is probably overly cautious : if the variable of the
 let-in is used, we consider the constructor to be dependent.
 If someday somebody feels it necessary, he/she could try to reduce
 the let-in's instead...

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16340 85f007b7-540e-0410-9357-904b9bb8a0f7