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

[Oliver Stannard via cfe-commits]

ostannard wrote:

That looks like the expected consequence of this change:
* Previously, the anonymous bitfield was erroneously given a 64-bit container, so it occupied 7 copies of the container type, starting at byte 8 of the struct and ending at 64.
* Now, it is given a 128-bit container, as required by AAPCS64, so it needs 4 copies of the container type, starting at byte 16 and ending at 80.

GCC also gives this struct a size of 80 bytes.

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