[DebugInfo][RemoveDIs] Allow for inserting DPValues at end() (#72379)

This trivial patch covers a bit of fallout from
https://reviews.llvm.org/D153990, where we moved the storage of trailing
DPValues into LLVMContext rather than being stored in each block. As a
result, you can now get a null DPMarker pointer from the end() iterator
where previously you didn't (and now it has to be explicitly created).

This is a sort-of stopgap measure -- there's another all-singing
all-dancing patch further down the line that refactors all of this so
that we don't allocate a DPMarker for every single Instruction. When
that lands this will all be refactored so that every time we request a
DPMarker, one is created if needs be. That's a performance-fix rather
than a functionality related patch though, so it'll come later.