[MLIR] Use a shared uniquer for affine maps and integer sets.

Affine maps and integer sets previously relied on a single lock for creating unique instances. In a multi-threaded setting, this lock becomes a contention point. This commit updates AffineMap and IntegerSet to use StorageUniquer instead. StorageUniquer internally uses sharded locks and thread-local caches to reduce contention. It is already used for affine expressions, types and attributes. On my local machine, this gives me a 5X speedup for an application that manipulates a lot of affine maps and integer sets.

This commit also removes the integer set uniquer threshold. The threshold was used to avoid adding integer sets with a lot of constraints to the hash_map containing unique instances, but the constraints and the integer set were still allocated in the same allocator and never freed, thus not saving any space expect for the hash-map entry.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D114942