Export the watchpoint related API (SBWatchpointLocation class and added SBTarget methods)

to the Python interface.

Implement yet another (threre're 3 now) iterator protocol for SBTarget: watchpoint_location_iter(),
to iterate on the available watchpoint locations.  And add a print representation for
SBWatchpointLocation.

Exercise some of these Python API with TestWatchpointLocationIter.py.

llvm-svn: 140595