`enum class : unsigned __int128` enumerator constants are truncated to `u64`

Input C/C++ Header

enum class E : unsigned __int128 { HEX = ~(unsigned __int128)0 };
int take(E e, int t);
unsigned sz(void);

Bindgen Invocation

$ bindgen input.h --no-rustfmt-bindings --enable-cxx-namespaces -- -x c++ -std=c++17

Actual Results

The generated type is correct (u128). The generated constant is u64::MAX, not u128::MAX:

pub const E_HEX: E = 18446744073709551615;
pub type E = u128;

C++ E::HEX is all-ones in both halves. Rust E_HEX has hi = 0.

take returns (hi != 0) + (lo != 0) + t. sizeof is 16 on both sides. bindgen exit 0, rustc exit 0:

C:    v=12 lo=18446744073709551615 hi=18446744073709551615 sz=16
Rust: v=11 lo=18446744073709551615 hi=0                    sz=16

build_builtin_ty already maps CXType_UInt128 → IntKind::U128. enum_val_unsigned() is Option<u64> via clang_getEnumConstantDeclUnsignedValue, so the high half is dropped while still emitting a typed u128 constant.

This is not #3416 (packed bitfield accessors shifting a 65-bit window in u64).

Expected Results

E_HEX should be ~(unsigned __int128)0 as u128 (all-ones), or bindgen should not emit a silently truncated value as a u128 constant.

Environment

bindgen current main: 77cbc72
bindgen:              0.73.1
clang:                Apple clang 21.0.0
rustc:                1.97.1
target:               aarch64-apple-darwin

Start by tracing enum constant generation through enum_val_unsigned() and compare it with build_builtin_ty(), which already maps CXType_UInt128 to IntKind::U128. Reproduce the issue with the provided C++ header and bindgen invocation, then verify that the generated E_HEX preserves both halves of the u128 value without silent truncation.