Cope with the case where the user-supplied callbacks want the watchpoint itself to be disabled!

Previously we put a WatchpointSentry object within StopInfo.cpp to disable-and-then-enable the watchpoint itself
while we are performing the actions associated with the triggered watchpoint, which can cause the user-initiated
watchpoint disabling action to be negated.

Add a test case to verify that a watchpoint can be disabled during the callbacks.

llvm-svn: 162483