Support blocklisting __BindgenBitfieldUnit

Simply skip emitting it if the list of blocklisted types/items matches
its name.

Also add a test that verifies blocklisting anonymous types by
`_bindgen_ty_*` works as expected.

Author: ian-h-chamberlain

bindgen-tests/tests/expectations/tests/anon_enum_blocklist.rs

@@ -0,0 +1,11 @@
+// bindgen-flags: --blocklist-type "_bindgen_ty_1"
+
+enum {
+  FLAG_X,
+  FLAG_Y,
+};
+
+enum {
+  FLAG_Z,
+  FLAG_W,
+};

bindgen-tests/tests/expectations/tests/blocklist_bitfield_unit.rs

@@ -0,0 +1,7 @@
+// bindgen-flags: --blocklist-type "__BindgenBitfieldUnit" --raw-line '#[path = "./struct_with_bitfields.rs"] mod bitfields;' --raw-line 'use bitfields::*;'
+struct C {
+  unsigned char x;
+  unsigned b1 : 1;
+  unsigned b2 : 1;
+  unsigned baz;
+};

bindgen-tests/tests/headers/anon_enum_blocklist.h

@@ -1,5 +1,7 @@
 //! Helpers for code generation that don't need macro expansion.
 
+use proc_macro2::{Ident, Span};
+
 use crate::ir::context::BindgenContext;
 use crate::ir::layout::Layout;
 
@@ -109,10 +111,13 @@ pub(crate) fn integer_type(
     Layout::known_type_for_size(ctx, layout.size)
 }
 
+pub(crate) const BITFIELD_UNIT: &str = "__BindgenBitfieldUnit";
+
 /// Generates a bitfield allocation unit type for a type with the given `Layout`.
 pub(crate) fn bitfield_unit(ctx: &BindgenContext, layout: Layout) -> syn::Type {
     let size = layout.size;
-    let ty = syn::parse_quote! { __BindgenBitfieldUnit<[u8; #size]> };
+    let bitfield_unit_name = Ident::new(BITFIELD_UNIT, Span::call_site());
+    let ty = syn::parse_quote! { #bitfield_unit_name<[u8; #size]> };
 
     if ctx.options().enable_cxx_namespaces {
         return syn::parse_quote! { root::#ty };

bindgen-tests/tests/headers/blocklist_bitfield_unit.h

@@ -5017,6 +5017,7 @@ pub(crate) fn codegen(
 }
 
 pub(crate) mod utils {
+    use super::helpers::BITFIELD_UNIT;
     use super::serialize::CSerialize;
     use super::{error, CodegenError, CodegenResult, ToRustTyOrOpaque};
     use crate::ir::context::BindgenContext;
@@ -5153,6 +5154,12 @@ pub(crate) mod utils {
         ctx: &BindgenContext,
         result: &mut Vec<proc_macro2::TokenStream>,
     ) {
+        if ctx.options().blocklisted_items.matches(BITFIELD_UNIT) ||
+            ctx.options().blocklisted_types.matches(BITFIELD_UNIT)
+        {
+            return;
+        }
+
         let bitfield_unit_src = include_str!("./bitfield_unit.rs");
         let bitfield_unit_src = if ctx.options().rust_features().min_const_fn {
             Cow::Borrowed(bitfield_unit_src)