[llvm] [clang] [RISCV] Remove experimental from Vector Crypto extensions (PR #74213)

[Alex Bradbury via cfe-commits]

asb wrote:

I failed to report back in the other review thread about the discussion at the previous RISC-V LLVM sync-up call. I think it's a fair summary to say we concluded was that gating the intrinsics on a flag was a good idea (though not a strict requirement if it was going to slow things down too much), and that we probably shouldn't agonise over the precise form this flag takes. What you propose here seems OK to me, but please add something to RISCVUsage indicating that in some cases an extension may be non-experimental but the intrinsics still experimental and `-menable-experimental-extensions` must be passed to Clang to use them. I also think a Clang release note might make sense, such as "It is now possible for intrinsics to be experimental while the extension itself is non-experimental. This is currently the case for the vector crypto extensions (Zvk*, Zvbb, Zvbc) where `-menable-experimental-extension` must still be passed to Clang in order to enable use of the C intrinsics."

https://github.com/llvm/llvm-project/pull/74213