[clang] [Cygwin] va_list must be treated like normal Windows (PR #143115)

[via cfe-commits]

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@@ -979,6 +979,9 @@ class LLVM_LIBRARY_VISIBILITY CygwinX86_64TargetInfo : public X86_64TargetInfo {
     if (Opts.CPlusPlus)
       Builder.defineMacro("_GNU_SOURCE");
   }
+  BuiltinVaListKind getBuiltinVaListKind() const override {
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jeremyd2019 wrote:

```suggestion

  BuiltinVaListKind getBuiltinVaListKind() const override {
```

Odd that clang-format doesn't trigger on that

https://github.com/llvm/llvm-project/pull/143115