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

[Joseph Huber via cfe-commits]

jhuber6 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.

I think `steady` has sufficient context here, (i.e. https://en.cppreference.com/w/cpp/chrono/steady_clock). And I mostly like it because it reads very similar to the existing `readcyclecounter`.

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