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

[John McCall via cfe-commits]

rjmccall wrote:

Okay, so x86_64 describes it in byte terms and says they're little-endian, which is consistent with the overall target.  Interestingly, it does not guarantee the content of the excess bits.  The code-generation in this patch is consistent with that: the extension we do is unnecessary but allowed, and then we truncate it away after load.  If we ever add some way to tell the backend that a truncation is known to be reversing a sign/zero-extension, we'll need to not set it on this target.

32-bit and 64-bit ARM describe it in terms of smaller units, but the units are expressly laid out according to the overall endianness of the target, which composes to mean that the bytes overall are also laid out according to that endianness.

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