[clang] [clang] Lower _BitInt(129+) to a different type in LLVM IR (PR #91364)

[Reid Kleckner via cfe-commits]

rnk wrote:

I played with the idea of using LLVM packed structs (`<{ i129 }>`) to represent something like this, but they don't work the way I expected them to do: https://godbolt.org/z/M6hMYYhax

LLVM DataLayout's idea of `sizeof(i129)` is still rounded up from 17 bytes to 32 bytes.

Using byte arrays for the in-memory type should work, so it's probably the best path forward.

https://github.com/llvm/llvm-project/pull/91364