[llvm] [AArch64] Prevent reuse of retired architecture names (PR #221782)

[via llvm-commits]

Lukacma wrote:

I am wondering if we could get away with simpler solution. This is only relevant to LLVM developers in the future and one of the requirements for patch being merged is passing testsuite. When we discussed this with @paulwalker-arm  a while ago, he proposed the idea of just adding negative tests for the retired features and registers. I think that is a nice simple solution for this. In the test we could just check that the errors/warnings are emitted when retired feature/register is used. This would prevent developers from readding them. What do you think ?

https://github.com/llvm/llvm-project/pull/221782