[ADT] Remove SFINAE constraint from llvm::iterator_range ctor for gcc-7

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which is an outdated, but supported compiler by llvm-project.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.
I tested GCC 8.4.0, and GCC 7.5.0, and now builds fine on both.

Differential Revision: https://reviews.llvm.org/D155441

Fixes https://github.com/llvm/llvm-project/issues/63843