Rollup merge of #116912 - compiler-errors:rustc_type_ir-bikeshedding, r=jackh726

Some renaming nits for `rustc_type_ir`

tl;dr:
* `ListTy` -> `Tys`
* `PlaceholderType` -> `PlaceholderTy`
* `RegionVid` -> `InferRegion`
* `ListBinderExistentialPredicate` -> `BoundExistentialPredicates`
* `GenericArgsRef` -> `GenericArgs`

r? jackh726

Author: fmease