`Not an item` panic for incomplete union

[adetaylor]

Input C/C++ Header

namespace {
namespace {
template <typename> union a { a b; };
} // namespace
} // namespace

This is nonsensical but results from reduction of a larger real codebase in google/autocxx#779, so is presumably this is the net result of some complex arrangement of templates.

It does compile OK with clang++:

$ cat <<HERE > test.cc
> namespace {
namespace {
template <typename> union a { a b; };
} // namespace
} // namespace
> HERE
$ clang++ -c test.cc 
$ clang++ --version
Debian clang version 13.0.1-+rc1-1~exp4
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Bindgen Invocation

$ git checkout ca78ae9c41998713db0c6fa70cd21089cdb2af2c # current master
$ cargo run -- test.hpp

Actual Results

thread 'main' panicked at 'Not an item: ItemId(3)', src/ir/context.rs:1464:21
stack backtrace:
   0: rust_begin_unwind
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/core/src/panicking.rs:107:14
   2: bindgen::ir::context::BindgenContext::resolve_item
             at ./src/ir/context.rs:1464:21
   3: bindgen::ir::context::BindgenContext::resolve_type
             at ./src/ir/context.rs:1428:9
   4: bindgen::ir::ty::Type::layout::{{closure}}
             at ./src/ir/ty.rs:232:21
   5: core::option::Option<T>::or_else
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/core/src/option.rs:1147:21
   6: bindgen::ir::ty::Type::layout
             at ./src/ir/ty.rs:219:9
   7: bindgen::ir::ty::Type::layout::{{closure}}
             at ./src/ir/ty.rs:232:21
   8: core::option::Option<T>::or_else
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/core/src/option.rs:1147:21
   9: bindgen::ir::ty::Type::layout
             at ./src/ir/ty.rs:219:9
  10: bindgen::ir::comp::CompInfo::each_known_field_layout
             at ./src/ir/comp.rs:1159:43
  11: bindgen::ir::comp::CompInfo::layout
             at ./src/ir/comp.rs:1115:9
  12: bindgen::ir::ty::Type::layout::{{closure}}
             at ./src/ir/ty.rs:221:43
  13: core::option::Option<T>::or_else
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/core/src/option.rs:1147:21
  14: bindgen::ir::ty::Type::layout
             at ./src/ir/ty.rs:219:9
  15: bindgen::ir::context::BindgenContext::compute_bitfield_units::{{closure}}
             at ./src/ir/context.rs:976:30
  16: bindgen::ir::context::BindgenContext::with_loaned_item
             at ./src/ir/context.rs:956:22
  17: bindgen::ir::context::BindgenContext::compute_bitfield_units
             at ./src/ir/context.rs:974:13
  18: bindgen::ir::context::BindgenContext::gen
             at ./src/ir/context.rs:1150:9
  19: bindgen::codegen::codegen
             at ./src/codegen/mod.rs:4289:5
  20: bindgen::Bindings::generate
             at ./src/lib.rs:2401:32
  21: bindgen::Builder::generate
             at ./src/lib.rs:1478:9
  22: bindgen::main::{{closure}}
             at ./src/main.rs:52:17
  23: std::panicking::try::do_call
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/std/src/panicking.rs:406:40
  24: __rust_try
  25: std::panicking::try
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/std/src/panicking.rs:370:19
  26: std::panic::catch_unwind
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/std/src/panic.rs:133:14
  27: bindgen::main
             at ./src/main.rs:51:34
  28: core::ops::function::FnOnce::call_once
             at /rustc/65f3f8b220f020e562c5dd848ff7319257a7ba45/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Expected Results

It probably doesn't make much sense to generate any bindings here, but this is currently preventing generation of other things in the same codebase.