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

[Matt Arsenault via llvm-commits]

================
@@ -312,6 +312,12 @@ void IntrinsicLowering::LowerIntrinsicCall(CallInst *CI) {
     CI->replaceAllUsesWith(ConstantInt::get(Type::getInt64Ty(Context), 0));
     break;
   }
+  case Intrinsic::readfixedtimer: {
+    errs() << "WARNING: this target does not support the llvm.readfixedtimer"
----------------
arsenm wrote:

I'm surprised we have this random printing here. Is IntrinicLowering even used? 

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