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

[Shilei Tian via cfe-commits]

https://github.com/shiltian commented:

Generally looks good to me. Just not sure about the name. "fixed timer" sounds pretty confusing to me. probably `readfixedfreqtimer`?

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