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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Added clang test and renamed to `readsteadycounter` as I think it's more descriptive and matches the existing `readcyclecounter` better.

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