Unwind: hoist placement delete into base class

Move the placement delete into the base class.  This permits the proper emission
of the virtual destructor in UnwindCursor by using the class specific placement
delete instead of the normal single element ::operator delete.  With this patch,
we can finally build libunwind as a DSO without a runtime dependency on
libc++/libc++abi.

llvm-svn: 228436