Work around Bindgen incorrect detection of nested name specifier

Summary:
Clang recently made major changes to the AST representation of nested name specifiers in [llvm-project#147835](llvm/llvm-project#147835). While waiting for Bindgen to be updated to understand the new representation, this diff changes our Bindgen invocation on `folly::IOBuf` to disregard the nested name `folly::IOBuf::Iterator`, which is unused anyway in the Rust `IOBuf` binding.

This unblocks staging builds for the LLVM Server Compiler team.

This diff causes the following difference in the Bindgen-generated Rust binding observed through `buck2 build fbcode//mode/opt fbcode//folly/rust/iobuf:iobuf-sys-bindgen`:

```lang=diff
@@ -571,8 +571,6 @@
         pub const IOBuf_CombinedOption_SEPARATE: root::folly::IOBuf_CombinedOption = 2;
         pub type IOBuf_CombinedOption = ::std::os::raw::c_int;
         pub type IOBuf_value_type = root::folly::ByteRange;
-        pub type IOBuf_iterator = root::folly::IOBuf_Iterator;
-        pub type IOBuf_const_iterator = root::folly::IOBuf_Iterator;
         pub type IOBuf_FreeFunction = ::std::option::Option<
             unsafe extern "C" fn(
                 buf: *mut ::std::os::raw::c_void,
@@ -765,31 +763,6 @@
                 "Offset of field: IOBuf::sharedInfo_",
             ][::std::mem::offset_of!(IOBuf, sharedInfo_) - 48usize];
         };
-        #[repr(C)]
-        #[derive(Debug, Copy, Clone)]
-        pub struct IOBuf_Iterator {
-            pub pos_: *const root::folly::IOBuf,
-            pub end_: *const root::folly::IOBuf,
-            pub val_: root::folly::ByteRange,
-        }
-        #[allow(clippy::unnecessary_operation, clippy::identity_op)]
-        const _: () = {
-            [
-                "Size of IOBuf_Iterator",
-            ][::std::mem::size_of::<IOBuf_Iterator>() - 32usize];
-            [
-                "Alignment of IOBuf_Iterator",
-            ][::std::mem::align_of::<IOBuf_Iterator>() - 8usize];
-            [
-                "Offset of field: IOBuf_Iterator::pos_",
-            ][::std::mem::offset_of!(IOBuf_Iterator, pos_) - 0usize];
-            [
-                "Offset of field: IOBuf_Iterator::end_",
-            ][::std::mem::offset_of!(IOBuf_Iterator, end_) - 8usize];
-            [
-                "Offset of field: IOBuf_Iterator::val_",
-            ][::std::mem::offset_of!(IOBuf_Iterator, val_) - 16usize];
-        };
     }
     pub mod facebook {
         #[allow(unused_imports)]
```

Bindgen issue: [bindgen#3264](rust-lang/rust-bindgen#3264)

Reviewed By: HighW4y2H3ll

Differential Revision: D80186965

fbshipit-source-id: 9f0546a2964b5d4e5c67e6bdd91b542e3e5f7b2c

Author: David Tolnay

folly/rust/iobuf/BUCK

@@ -48,6 +48,12 @@ rust_bindgen_library(
         "facebook::rust::.*",
     ],
     blocklist_types = [
+        # Work around incorrect detection of nested name specifier. T234253370
+        # Bindgen needs to be fixed to understand the new AST representation.
+        # https://github.com/llvm/llvm-project/pull/147835
+        "folly::IOBuf_Iterator",
+        "folly::IOBuf_iterator",
+        "folly::IOBuf_const_iterator",
         # These seems to come from type_traits / make_signed via fbvector
         "folly::fbvector.*",
         "__type",