Switch SBBreakpoint to storing a weak_ptr of the internal breakpoint object

Summary:
There is nothing we can do with the breakpoint once the associated
target becomes deleted. This will make sure we don't hold on to more
resources than we need in this case. In particular, this fixes the case
TestStepOverBreakpoint on windows, where a lingering SBBreakpoint object
causes us to nor unmap the executable file from memory.

Reviewers: clayborg, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D30249

llvm-svn: 296328