[llvm] [mlir] [ADT] Reduce boilerplate in DenseSet (NFC) (PR #158456)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> Could we do something else and retain the `DenseSet`/`SmallDenseSet` classes so that we can still use forward declaration and have nicer named produced by tooling?

@kuhar I've put in one more layer.  A type alias eliminates the boiler plate.  The user facing "forwarding" class allows us to keep the forward declaration in mlir as is.  We still manage to reduce the total line count.


https://github.com/llvm/llvm-project/pull/158456