[Clang][Cygwin] Use correct BuiltinVaListKind.

This was in older patches for Cygwin support, but was somehow lost in
the latest rounds.

Author: jeremyd2019

clang/lib/Basic/Targets/X86.h

@@ -984,6 +984,10 @@ class LLVM_LIBRARY_VISIBILITY CygwinX86_64TargetInfo : public X86_64TargetInfo {
     this->WCharType = TargetInfo::UnsignedShort;
   }
 
+  BuiltinVaListKind getBuiltinVaListKind() const override {
+    return TargetInfo::CharPtrBuiltinVaList;
+  }
+
   void getTargetDefines(const LangOptions &Opts,
                         MacroBuilder &Builder) const override {
     X86_64TargetInfo::getTargetDefines(Opts, Builder);