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).
So do this only in known-bad cases.

Fixes #2244

Author: emilio

src/ir/ty.rs

@@ -1031,7 +1031,16 @@ impl Type {
                 CXType_ObjCObjectPointer |
                 CXType_MemberPointer |
                 CXType_Pointer => {
-                    let pointee = ty.pointee_type().unwrap();
+                    let mut pointee = ty.pointee_type().unwrap();
+                    if *ty != canonical_ty {
+                        let canonical_pointee =
+                            canonical_ty.pointee_type().unwrap();
+                        // clang sometimes loses pointee constness here, see
+                        // #2244.
+                        if canonical_pointee.is_const() != pointee.is_const() {
+                            pointee = canonical_pointee;
+                        }
+                    }
                     let inner =
                         Item::from_ty_or_ref(pointee, location, None, ctx);
                     TypeKind::Pointer(inner)

tests/expectations/tests/pointer-attr.rs

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