[clang] [llvm] [ARM][AArch64] Introduce the Armv9.8-A architecture version (PR #229361)

[Jonathan Thackray via cfe-commits]

jthackray wrote:

> Drive by comment - Could there be a clang release note too? Do we usually add those?

Sure, done. (We've not been consistent; we've added for new CPUs but not for architecture. It's a significant enough change it should be in both release notes).

https://github.com/llvm/llvm-project/pull/229361