Items losing type parameters over course of typechecking pass.

[froystig]

Fix the issue revealed in run-pass/generic-fn-twice.rs. Functions in modules (perhaps more generally items in items) appear to lose their recollection of having any type parameters after their first type unification.

[pcwalton]

New typechecker fixes this. Un-XFAILed as of revision 35e73b0.