Change the btree algorithm to split nodes bottom-up instead of top-down.

Change the btree algorithm to split nodes bottom-up instead of top-down.
This results in an (IMO) simpler algorithm, results in fewer splits, and
is more amenable to delta handling (there is no reason to mutate the tree
at all when adding a delta to a position that already exists in the tree).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49609 91177308-0d34-0410-b5e6-96231b3b80d8