[clang] Revert "[X86][clang] Lift _BitInt() supported max width." (PR #81175)

[Harald van Dijk via cfe-commits]

hvdijk wrote:

> At least that shouldn't be a problem: LLVM has separate concepts of "store size" and "alloc size" where only the latter rounds up to alignment. So `load i129` is specified to access only 9 bytes, not 16 bytes.

Sure, but when it appears inside a struct, the memory reserved is based on the alloc size, not the store size, see `StructLayout`. That applies even for a packed struct.

https://github.com/llvm/llvm-project/pull/81175