[mlir] Make use of C++17 language features

Now that C++17 is enabled in LLVM, a lot of the TODOs and patterns to emulate C++17 features can be eliminated.
The steps I have taken were essentially:
```
git grep C++17
git grep c++17
git grep "initializer_list<int>"
```
and address given comments and patterns.
Most of the changes boiled down to just using fold expressions rather than initializer_list.

While doing this I also discovered that Clang by default restricts the depth of fold expressions to 256 elements. I specifically hit this with `TestDialect` in `addOperations`. I opted to not replace it with fold expressions because of that but instead adding a comment documenting the issue.
If any other functions may be called with more than 256 elements in the future we might have to revert other parts as well.
I don't think this is a common occurence besides the `TestDialect` however. If need be, this could potentially be fixed via `mlir-tblgen` in the future.

Differential Revision: https://reviews.llvm.org/D131323