Generalize ownership/passing semantics to allow dsymutil to own abbreviations via...

Generalize ownership/passing semantics to allow dsymutil to own abbreviations via unique_ptr

While still allowing CodeGen/AsmPrinter in llvm to own them using a bump
ptr allocator. (might be nice to replace the pointers there with
something that at least automatically calls their dtors, if that's
necessary/useful, rather than having it done explicitly (I think a typed
BumpPtrAllocator already does this, or maybe a unique_ptr with a custom
deleter, etc))

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253409 91177308-0d34-0410-b5e6-96231b3b80d8