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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Are we assuming any particular relationship to __builtin_readcyclecounter in terms of scales etc? 

__builtin_readsteadycounter could be used to access x86 MPERF clock counters, but to access the corresponding APERF clock we'd then need a __builtin_readvariablecounter equivalent (__builtin_readcyclecounter gives the separate RDTSC clock value)

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