add a new method, play around with some code.

add a new method, play around with some code.

Fix a *bug* in the extendIntervalEndTo method.  In particular, if adding
[2:10) to an interval containing [0:2),[10:30), we produced [0:10),[10,30).
Which is not the most smart thing to do.  Now produce [0:30).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23841 91177308-0d34-0410-b5e6-96231b3b80d8