Remove OpTrait, AttrTrait and TypeTrait

  - Remove the `{Op,Attr,Type}Trait` TableGen classes and replace with `Trait`
  - Rename `OpTraitList` to `TraitList` and use it in a few places

The bulk of this change is a mechanical s/OpTrait/Trait/ throughout the codebase.

Reviewed By: rriddle, jpienaar, herhut

Differential Revision: https://reviews.llvm.org/D118543