Add test case with randomly ordered insertions, massive coalescing.

Add test case with randomly ordered insertions, massive coalescing.

Implement iterator::erase() in a simple version that erases nodes when they
become empty, but doesn't try to redistribute elements among siblings for better
packing.

Handle coalescing across leaf nodes which may require erasing entries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120226 91177308-0d34-0410-b5e6-96231b3b80d8