[llvm] [RemoveDIs][DebugInfo] Make DIAssignID always replaceable (PR #78300)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> This LGTM, presumably it works just as well in the non-remove-DIs mode? (Which needs to keep working for a decent period of time).

It does, the only related change is the removal of the "rauw for mav" part at the start for `at::rauw` - but that's redundant now that we actually call rauw on the DIAssignID directly.

https://github.com/llvm/llvm-project/pull/78300