Add IntervalMap::iterator::set{Start,Stop,Value} methods that allow limited

Add IntervalMap::iterator::set{Start,Stop,Value} methods that allow limited
editing of the current interval.

These methods may cause coalescing, there are corresponding set*Unchecked
methods for editing without coalescing. The non-coalescing methods are useful
for applying monotonic transforms to all keys or values in a map without
accidentally coalescing transformed and untransformed intervals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120829 91177308-0d34-0410-b5e6-96231b3b80d8