[llvm] [AArch64] Rename VariantKind to Specifier (PR #132595)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks for the feedback. I decide to keep the current `VK_*` spelling. It has been long-standing and keeping the name makes `git log -S VK_GOT` useful :)

https://github.com/llvm/llvm-project/pull/132595