[clang] [llvm] [LLVM] Add `__builtin_readsteadycounter` intrinsic and builtin for realtime clocks (PR #81331)

[Artem Belevich via cfe-commits]

https://github.com/Artem-B commented:

LGTM with few nits for general and NVPTX parts.

https://github.com/llvm/llvm-project/pull/81331