Change ty_with_args to return Ty instead of Result

Although, we would like to avoid crashes whenever
possible, and that's why I wanted to make this API fallible. It's
looking pretty hard to do proper validation.

I think many of our APIs will unfortunately depend on the user doing
the correct thing since at the MIR level we are working on,
we expect types to have been checked already.

compiler/stable_mir/src/compiler_interface.rs

@@ -91,7 +91,7 @@ pub trait Context {
     fn def_ty(&self, item: DefId) -> Ty;
 
     /// Returns the type of given definition instantiated with the given arguments.
-    fn def_ty_with_args(&self, item: DefId, args: &GenericArgs) -> Result<Ty, Error>;
+    fn def_ty_with_args(&self, item: DefId, args: &GenericArgs) -> Ty;
 
     /// Returns literal value of a const as a string.
     fn const_literal(&self, cnst: &Const) -> String;