[clang] [ARM, AArch64] Fix ABI bugs with over-sized bitfields (PR #126774)

[Florian Mayer via cfe-commits]

fmayer wrote:

Could we make sure this gets called out in the Release Notes? Even if it's a fix, this is a change of ABI which could break stuff.

https://github.com/llvm/llvm-project/pull/126774