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

[John McCall via cfe-commits]

rjmccall wrote:

Given all that, I feel pretty comfortable relying on using LLVM's `i96` stores and so on.  I do worry some that we're eventually going to run into a target where the `_BitInt` ABI does not match what LLVM wants to generate for `i96` load/store, but we should be able to generalize this so that targets can override the `_BitInt` operations pretty easily.

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