Add an SB API SBFrame::WatchValue() and exported to the Python interface to

set a watchpoint Pythonically.  If the find-and-watch-a-variable operation
fails, an invalid SBValue is returned, instead.

Example Python usage:

        value = frame0.WatchValue('global',
                                  lldb.eValueTypeVariableGlobal,
                                  lldb.LLDB_WATCH_TYPE_READ|lldb.LLDB_WATCH_TYPE_WRITE)

Add TestSetWatchpoint.py to exercise this API.
We have 400 test cases now.

llvm-svn: 140436