It turns out the SFINAE constraint breaks building MLIR using GCC-7,
which is an outdated, but supported compiler by llvm-project.
I tried to find a solution for fixing it, but I decided to cut branches
and just simply remove the SFINAE constraint until we drop GCC-7.
It was originally introduced by D152891.
Allegedly, GCC-8 and above builds just fine.
Depends on D152891