[clang] [CIR] Emit _BitInt constants in memory form (PR #205605)

[via cfe-commits]

adams381 wrote:

Superseded by #206606, which takes the approach you pointed to, @andykaylor: keep the literal `_BitInt` in CIRGen and widen + sign/zero-extend to the storage integer during lowering, where `bool` becomes `i8`. That fixes the undefined padding bits (`signed _BitInt(6) = -1` now writes `0xff`, not `0x3f`) and also covers loads, stores, struct/array members, and zero/undef globals. Closing in favor of that PR.

https://github.com/llvm/llvm-project/pull/205605