Handle bang functions correctly in typestate

The logic for how the "returns" constraint was handled was always
dodgy, for reasons explained in the comments I added to
auxiliary::fn_info in this commit. Fixed it by adding distinct
"returns" and "diverges" constraints for each function, which
are both handled positively (that is: for a ! function, the
"diverges" constraint must be true on every exit path; for
any other function, the "returns" constraint must be true
on every exit path).

Closes #779

src/test/compile-fail/bang-tailexpr.rs

@@ -0,0 +1,4 @@
+// xfail-stage0
+// error-pattern: some control paths may return
+fn f() -> ! { 3 }
+fn main(){}