[clang] [clang] Remove isOSWindows() checks (PR #129909)

[Reid Kleckner via cfe-commits]

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@@ -4769,31 +4769,32 @@ ExprResult Sema::CheckOSLogFormatStringArg(Expr *Arg) {
 /// Check that the user is calling the appropriate va_start builtin for the
 /// target and calling convention.
 static bool checkVAStartABI(Sema &S, unsigned BuiltinID, Expr *Fn) {
-  const llvm::Triple &TT = S.Context.getTargetInfo().getTriple();
+  const TargetInfo &TI = S.Context.getTargetInfo();
+  bool IsMicrosoftABI = TI.getCXXABI().isMicrosoft();
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rnk wrote:

This seems like it would potentially be an incorrect behavior change for mingw. cc @mstorsjo 

https://github.com/llvm/llvm-project/pull/129909