ty: Use canonical type to access pointee.

Using the canonical type fixes this but changes the output of some tests
(in particular, pointer to typedefs now point to the underlying type).

That seems somewhat acceptable tho, and shouldn't be a correctness
regression (so this patch should be a correctness improvement).

Fixes #2244

Author: emilio

src/ir/ty.rs

@@ -1031,7 +1031,7 @@ impl Type {
                 CXType_ObjCObjectPointer |
                 CXType_MemberPointer |
                 CXType_Pointer => {
-                    let pointee = ty.pointee_type().unwrap();
+                    let pointee = canonical_ty.pointee_type().unwrap();
                     let inner =
                         Item::from_ty_or_ref(pointee, location, None, ctx);
                     TypeKind::Pointer(inner)

tests/expectations/tests/call-conv-typedef.rs

@@ -0,0 +1 @@
+void a(const char __attribute__((btf_type_tag("a"))) *);