even when keys get RAUWed and deleted during its lifetime. By default the keys
act like WeakVHs, but users can pass a third template parameter to configure
how updates work and whether to do anything beyond updating the map on each
action.
It's also possible to automatically acquire a lock around ValueMap updates
triggered by RAUWs and deletes, to support the ExecutionEngine.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84890 91177308-0d34-0410-b5e6-96231b3b80d8
