[clang] [CIR] Implement__builtin_va_arg (PR #153834)

[Henrich Lauko via cfe-commits]

================
@@ -3415,4 +3415,126 @@ def CIR_FAbsOp : CIR_UnaryFPToFPBuiltinOp<"fabs", "FAbsOp"> {
   }];
 }
 
+//===----------------------------------------------------------------------===//
+// Variadic Operations
+//===----------------------------------------------------------------------===//
+
+def CIR_VAStartOp : CIR_Op<"va.start"> {
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xlauko wrote:

To be more consistent in naming:

```suggestion
def CIR_VAStartOp : CIR_Op<"va_start"> {
```

and update its uses elsewhere.

https://github.com/llvm/llvm-project/pull/153834