Switch SBWatchpoint::SetEnabled over to using Process::{Enable,Disable}Watchpoint.

We don't have a good story for what happens to watchpoints when you don't
have a process, or if your process exits.  Clearing that up will instruct 
how to fix this for real.

Also added a test to make sure disable->enable works as well.
This resolves llvm.org/pr30789.

llvm-svn: 285742