[Clang] `alignof(nullptr_t)` is inconsistent with MSVC when using MSVC ABI

Currently, Clang sets `alignof(nullptr_t)` equal to `alignof(void*)` when using MSVC ABI, but MSVC sets `alignof(nullptr_t)` to 1. [Godbolt link](https://godbolt.org/z/qj6TPWf3v). It seems that if one explicitly uses `alignof(nullptr_t)` or `alignas(nullptr_t)`, there would be source and/or ABI incompatibility with MSVC.

I think Clang's behavior is desired and consistent with C23 requirements (see also [CWG2966](https://cplusplus.github.io/CWG/issues/2966.html)). Also, MSVC seemingly treats `nullptr_t` as something aligned as `void*` when calculating class layout, which is a bit weird.

Do we want to get closer to MSVC or inform MSVC to change?

Start by reproducing the alignment difference using the linked Godbolt example, then read CWG2966 and compare the reported Clang and MSVC behavior. Done requires a project decision on whether Clang should preserve its current behavior, change toward MSVC, or pursue coordination with MSVC; the issue does not identify implementation files or tests.

Written by the indexing model from the issue text.