[clang] [llvm] [AArch64] Enable FEAT_SVE2p1 by default for Armv9.4-A and later (PR #120753)

[Jonathan Thackray via llvm-commits]

jthackray wrote:

> Actually this is being added as a mandatory dependency of 9.4, should it not be added as one of the default features instead?

Good point. I'll amend.

https://github.com/llvm/llvm-project/pull/120753