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

This patch is a necessary step to allowing the new non-intrinsic debug
info to replace llvm.dbg.assign intrinsics. DIAssignIDs must be able to
look up the debug assigns that refer to them, and this patch makes them
always be considered "replaceable", allowing us to track and replace uses
for non-temporary instances.