[mlir][StorageUniquer] Refactor parametric storage to use sharded dense sets

[mlir][StorageUniquer] Refactor parametric storage to use sharded dense sets

This revisions implements sharding in the storage of parametric instances to decrease lock contention by sharding out the allocator/mutex/etc. to use for a specific storage instance based on the hash key. This is a somewhat common approach to reducing lock contention on data structures, and is used by the concurrent hashmaps provided by folly/java/etc. For several compilations tested, this removed all/most lock contention from profiles and reduced compile time by several seconds.

Differential Revision: https://reviews.llvm.org/D89659