[clang] [CIR] Implement missing __sync_* atomic builtins (PR #214606)

[Sirui Mu via cfe-commits]

================
@@ -2063,11 +2120,19 @@ RValue CIRGenFunction::emitBuiltinExpr(const GlobalDecl &gd, unsigned builtinID,
   case Builtin::BI__sync_xor_and_fetch:
   case Builtin::BI__sync_nand_and_fetch:
   case Builtin::BI__sync_val_compare_and_swap:
+    return RValue::get(
----------------
Lancern wrote:

The OGCG contains an `llvm_unreachable` here instead. Looks like sema would transform all `__sync_val_compare_and_swap` to `__sync_val_compare_and_swap_2/4/8/16`. Let's follow the OGCG's approach.

https://github.com/llvm/llvm-project/pull/214606