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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

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

Naming is the hard part. I was also thinking about `readrealtimecounter` or something. Maybe `readsteadycounter`?

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