IR: Move MDNode operands from the back to the front

Having the operands at the back prevents subclasses from safely adding
fields.  Move them to the front.

Instead of replicating the custom `malloc()`, `free()` and `DestroyFlag`
logic that was there before, overload `new` and `delete`.

I added calls to a new `GenericMDNode::dropAllReferences()` in
`LLVMContextImpl::~LLVMContextImpl()`.  There's a maze of callbacks
happening during teardown, and this resolves them before we enter
the destructors.

Part of PR21532.