Add a new RewriteRope data structure which is a smarter way to represent the text

Add a new RewriteRope data structure which is a smarter way to represent the text
backing a rewrite buffer than using an std::vector<char>.  This class was hacked
together very quickly and needs to be cleaned up, but it seems to work.  It speeds
up rewriting a a 7M file from 6.43s to 0.24s on my machine.  The impl could also
be made to be a lot more algorithmically sound.

This produces identical output to using vector on this testcase, if it causes a
problems or bugs are encountered, it can be disabled by changing the
RewriteBuffer::Buffer typedef back.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@43884 91177308-0d34-0410-b5e6-96231b3b80d8