Fix layout with packed class that only has a vtable

I noticed that a `#pragma packed` class that has only a vtable but no
data members is not marked repr(packed).  The bug is that is_packed
examines data members but not the vtable when applying its heuristic.

Author: tromey

src/ir/comp.rs

@@ -1517,6 +1517,10 @@ impl CompInfo {
             }) {
                 info!("Found a struct that was defined within `#pragma packed(...)`");
                 return true;
+            } else if self.has_own_virtual_method {
+                if parent_layout.align == 1 {
+                    return true;
+                }
             }
         }
 

tests/expectations/tests/packed-vtable.rs

@@ -0,0 +1,39 @@
+/* automatically generated by rust-bindgen */
+
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+#![cfg(feature = "nightly")]
+
+#[repr(C)]
+pub struct PackedVtable__bindgen_vtable(::std::os::raw::c_void);
+#[repr(C, packed)]
+#[derive(Debug)]
+pub struct PackedVtable {
+    pub vtable_: *const PackedVtable__bindgen_vtable,
+}
+#[test]
+fn bindgen_test_layout_PackedVtable() {
+    assert_eq!(
+        ::std::mem::size_of::<PackedVtable>(),
+        8usize,
+        concat!("Size of: ", stringify!(PackedVtable))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<PackedVtable>(),
+        1usize,
+        concat!("Alignment of ", stringify!(PackedVtable))
+    );
+}
+impl Default for PackedVtable {
+    fn default() -> Self {
+        unsafe { ::std::mem::zeroed() }
+    }
+}
+extern "C" {
+    #[link_name = "\u{1}_ZN12PackedVtableD1Ev"]
+    pub fn PackedVtable_PackedVtable_destructor(this: *mut PackedVtable);
+}

tests/headers/packed-vtable.h

@@ -0,0 +1,10 @@
+// bindgen-flags: --raw-line '#![cfg(feature = "nightly")]' --rust-target 1.33 -- -x c++ -std=c++11
+
+#pragma pack(1)
+
+// This should be packed.
+struct PackedVtable {
+  virtual ~PackedVtable();
+};
+
+#pragma pack()