Add ADT/IntervalMap.

This is a sorted interval map data structure for small keys and values with
automatic coalescing and bidirectional iteration over coalesced intervals.

Except for coalescing intervals, it provides similar functionality to std::map.
It is however much more compact for small keys and values, and hopefully faster
too.

The container object itself can hold the first few intervals without any
allocations, then it switches to a cache conscious B+-tree representation. A
recycling allocator can be shared between many containers, even between
containers holding different types.

The IntervalMap is initially intended to be used with SlotIndex intervals for:

- Backing store for LiveIntervalUnion that is smaller and faster than std::set.

- Backing store for LiveInterval with less overhead than std::vector for typical
  intervals and O(N log N) merging of large intervals. 99% of virtual registers
  need 4 entries or less and would benefit from the small object optimization.

- Backing store for LiveDebugVariable which doesn't exist yet, but will track
  debug variables during register allocation.

This is a work in progress. Missing items are:

- Performance metrics.
- erase().
- insert() shrinkage.
- clear().
- More performance metrics.
- Simplification and detemplatization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119787 91177308-0d34-0410-b5e6-96231b3b80d8

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