[clang] [clang] Fix inaccurate wording of warn_second_arg_of_va_start_not_last_named_param (PR #131238)

[Erich Keane via cfe-commits]

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@@ -4893,7 +4893,7 @@ bool Sema::BuiltinVAStart(unsigned BuiltinID, CallExpr *TheCall) {
 
   if (!SecondArgIsLastNamedArgument)
     Diag(TheCall->getArg(1)->getBeginLoc(),
-         diag::warn_second_arg_of_va_start_not_last_named_param);
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erichkeane wrote:

I would have preferred we change the name of the control variable (`SecondArgIsLastNamedArg`) as well/as a part of this.

https://github.com/llvm/llvm-project/pull/131238