C2Rust panics in `translator/simd.rs` on `__builtin_shufflevector` with `vector_size(16)` vector typedef

[rikki322]

C2Rust crashes when transpiling a valid C program that uses the GCC/Clang vector_size extension together with __builtin_shufflevector.
This appears related to incomplete vector/SIMD support, but unlike earlier failures such as unsupported vector initializer lists, this case triggers an internal panic in translator/simd.rs.
Reproducer

typedef int v4si __attribute__((vector_size(16)));

int main(void) {
    v4si a = {1, 2, 3, 4};
    v4si b = {5, 6, 7, 8};
    v4si c = __builtin_shufflevector(a, b, 3, 1, 2, 0);
    return c[0];
}

Observed Behavior
C2Rust reports unsupported vector type handling and then panics:

Transpiling test.c
error: Skipping declaration Some(Typedef { name: "v4si", typ: CQualTypeId { qualifiers: Qualifiers { is_const: false, is_restrict: false, is_volatile: false }, ctype: CTypeId(47) }, is_implicit: false, target_dependent_macro: None }) due to error: Unsupported type Vector(CQualTypeId { qualifiers: Qualifiers { is_const: false, is_restrict: false, is_volatile: false }, ctype: CTypeId(31) }, 4)

thread 'main' panicked at 'internal error: entered unreachable code: Found something other than a cast cast: DeclRef(CQualTypeId { qualifiers: Qualifiers { is_const: false, is_restrict: false, is_volatile: false }, ctype: CTypeId(60) }, CDeclId(76), LValue)', c2rust-transpile/src/translator/simd.rs:496:22
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The input program is valid C using GCC/Clang vector extensions.
Even if this vector/SIMD feature is not yet supported, C2Rust should fail gracefully with a structured error, rather than panicking with entered unreachable code.
Environment

• c2rust version: v0.21.0
• platform: Ubuntu 24.04
• Rust: nightly-2022-08-08
• Clang version: 17.0.6