[clang] [Clang] Amend SME attributes with support for ZT0. (PR #77941)

[Sander de Smalen via cfe-commits]

sdesmalen-arm wrote:

[rebased patch on top of #78424]

With the contentious part of adding new bits addressed in #78424 and @rsandifo-arm having reviewed and informally accepted the Arm/ZT0 side of the changes in this pull-request, are you happy to formally accept this PR @erichkeane / @AaronBallman?

https://github.com/llvm/llvm-project/pull/77941