[clang] [C23] Add __builtin_c23_va_start (PR #131166)

[Erich Keane via cfe-commits]

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@@ -4864,14 +4877,22 @@ bool Sema::BuiltinVAStart(unsigned BuiltinID, CallExpr *TheCall) {
     return true;
 
   // Verify that the second argument to the builtin is the last argument of the
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erichkeane wrote:

```suggestion
  // Verify that the second argument to the builtin is the last non-variadic argument of the
```

right?  Alternatively 'named', though that isn't really accurate.

https://github.com/llvm/llvm-project/pull/131166