[clang] [clang] Match MSVC ABI for over-aligned base tail padding on Arm64 (PR #210461)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> Please check that this interacts with the planned changes in #208256 in a reasonable way.
> 
> Otherwise LGTM

This LGTM too. Changes in  #208256 are orthogonal concerns. The latter PR  preserves specific type alignments under `#pragma pack`, this PR tracks natural vs explicit over-alignment for `Arm64` layout. There will be some textual conflicts in `getAdjustedElementInfo` and `finalizeLayout`.  I suggest this PR be landed first. Then #208256.

https://github.com/llvm/llvm-project/pull/210461