Change the RewriteRope::Chunks data structure from an std::list into

Change the RewriteRope::Chunks data structure from an std::list into
a nice shiny B+ Tree variant.  This fixes the last of the known algorithmic
issues with the rewriter, allowing a significant speedup.  For example,
-emit-html on Ted's 500K .i file speeds up from 26.8s -> 0.64s in a
debug build (41x!) and 5.475s -> 0.132s (41x!) in an optimized build.

This code is functional but needs to be cleaned up, ifdefs removed, better
commented, and moved to a .cpp file.  I plan to do this tomorrow.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49635 91177308-0d34-0410-b5e6-96231b3b80d8