IR: Update references to temporaries before deleting

IR: Update references to temporaries before deleting

During `MDNode::deleteTemporary()`, call `replaceAllUsesWith(nullptr)`
to update all tracking references to `nullptr`.

This fixes PR22280, where inverted destruction order between tracking
references and the temporaries themselves caused a use-after-free in
`LLParser`.

An alternative fix would be to add an assertion that there are no users,
and continue to fix inverted destruction order in clients (like
`LLParser`), but instead I decided to make getting-teardown-right easy.
(If someone disagrees let me know.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226866 91177308-0d34-0410-b5e6-96231b3b80d8