[mlir][llvm] Fix compiler error on GCC 9

This patch fixes the following compiler error:

    error: declaration of ‘mlir::LLVM::cconv::CConv mlir::LLVM::detail::CConvAttrStorage::CConv’ changes meaning of ‘CConv’ [-fpermissive]

CConv as a member variable name was shadowing CConv as an enumeration,
hence the compiler error.

Reviewed By: ftynse, alexbatashev

Differential Revision: https://reviews.llvm.org/D126530