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.

llvm-svn: 120829