LLVMABIAlignmentOfType(i128) reports 16 on this target, however the C
ABI uses align(4).

Clang in LLVM 17 does this:

%struct.foo = type { i32, i128 }

Clang in LLVM 18 does this:

%struct.foo = type <{ i32, i128 }>

Clang is working around the 16-byte alignment to use align(4) for the C
ABI by making the LLVM struct packed.