Do an initial hack at replacing one of the incredibly inefficient

Do an initial hack at replacing one of the incredibly inefficient
(but simple!) datastructures in the rewriter with a more complex but
more efficient one.

This replaces the Deltas vector with a specialized BTree that makes
delta lookups much more efficient.  This speeds up -emit-html on a 500K
.i file from 157.154 to 27.127 seconds on my machine (5.8x).

While this code is functional, it isn't very pretty, I have much
refactoring planned for it, and will remove the USE_VECTOR ifdef.
Stay tuned.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49586 91177308-0d34-0410-b5e6-96231b3b80d8