Add FieldInfo::field_type_name

The relevance of making a ield private may depend on a field's type.
Some fields should be protected against manipulation by Rust code for
the same reason `Vec::set_len` is `unsafe`.

bindgen/callbacks.rs

@@ -205,4 +205,6 @@ pub struct FieldInfo<'a> {
     pub type_name: &'a str,
     /// The name of the field.
     pub field_name: &'a str,
+    /// The name of the type of the field.
+    pub field_type_name: Option<&'a str>,
 }

bindgen/codegen/mod.rs

@@ -1515,6 +1515,7 @@ impl<'a> FieldCodegen<'a> for FieldData {
                 cb.field_visibility(FieldInfo {
                     type_name: &parent_item.canonical_name(ctx),
                     field_name,
+                    field_type_name: field_ty.name(),
                 })
             }),
             self.annotations(),
@@ -1889,6 +1890,7 @@ impl<'a> FieldCodegen<'a> for Bitfield {
 
         let bitfield_ty_item = ctx.resolve_item(self.ty());
         let bitfield_ty = bitfield_ty_item.expect_type();
+        let bitfield_ty_ident = bitfield_ty.name();
 
         let bitfield_ty_layout = bitfield_ty
             .layout(ctx)
@@ -1916,6 +1918,7 @@ impl<'a> FieldCodegen<'a> for Bitfield {
                 cb.field_visibility(FieldInfo {
                     type_name: &parent_item.canonical_name(ctx),
                     field_name,
+                    field_type_name: bitfield_ty_ident,
                 })
             })
         });