sess: disable polymorphisation

This commit disables polymorphisation to resolve regressions related to
closures which inherit unused generic parameters and are then used in
casts or reflection.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/polymorphization/const_parameters/functions.rs

@@ -1,4 +1,5 @@
 // build-fail
+// compile-flags:-Zpolymorphize=on
 #![feature(const_generics, rustc_attrs)]
 //~^ WARN the feature `const_generics` is incomplete