Add a commnad to set a condition for a watchpoint. Example:

    watchpoint modify -c 'global==5'

modifies the last created watchpoint so that the condition expression
is evaluated at the stop point to decide whether we should proceed with
the stopping.

Also add SBWatchpont::SetCondition(const char *condition) to set condition
programmatically.

Test cases to come later.

llvm-svn: 142227