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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> What do you think of renaming getVariantKindName? Otherwise looks good to me.

Thanks. Done. Updated some comments as well.

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