[Transforms] Fix -Wunused-variable in ExpandVariadics.cpp (NFC)

/llvm-project/llvm/lib/Transforms/IPO/ExpandVariadics.cpp:426:14:
error: unused variable 'OriginalFunctionIsDeclaration' [-Werror,-Wunused-variable]
  const bool OriginalFunctionIsDeclaration = OriginalFunction->isDeclaration();
             ^
/llvm-project/llvm/lib/Transforms/IPO/ExpandVariadics.cpp:445:13:
error: unused variable 'VariadicWrapperDefine' [-Werror,-Wunused-variable]
  Function *VariadicWrapperDefine =
            ^
2 errors generated.