[clang] [clang] Generate appropriate assume in presence of libc's memcpy (PR #119704)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> This is no longer the case after [N3322](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3322.pdf).
> 
> (Incidentally, I wrote a blog post about this yesterday: [developers.redhat.com/articles/2024/12/11/making-memcpynull-null-0-well-defined](https://developers.redhat.com/articles/2024/12/11/making-memcpynull-null-0-well-defined))

Nice! The next step might be to update glibc string.h `__nonnull`

https://github.com/llvm/llvm-project/pull/119704