Don't call DisableBreakpointSite (i.e. don't try to remove the breakpoint from the target process) if the target
process is no longer alive.

<rdar://problem/13320991>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178936 91177308-0d34-0410-b5e6-96231b3b80d8