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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

New intrinsic sounds right - a constant frequency counter is a different thing to a variable frequency counter.

"Steady" implies unchanging, so I'd agree with `readfixedfreqtimer` or similar.

We can't have a ratio between the two counters since one changes frequency and one doesn't.

Does x64 have something that maps usefully onto a fixed frequency counter intrinsic?

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