Add a Briggs and Torczon sparse set implementation.

For objects that can be identified by small unsigned keys, SparseSet
provides constant time clear() and fast deterministic iteration. Insert,
erase, and find operations are typically faster than hash tables.

SparseSet is useful for keeping information about physical registers,
virtual registers, or numbered basic blocks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151110 91177308-0d34-0410-b5e6-96231b3b80d8

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