[RemoveDIs] Do not load into new debug info format from bitcode by default (#86268)

This is NFC right now, as the global default behaviour is also "do not
load into the new debug info format by default", but we want to change
that soon.

Additionally unconditionally convert from the new debug info format into
if we've loaded into it (e.g., if the bitcode file loaded was already in
the new format).

The latter change is needed because verify-uselistorder doesn't yet
understand DbgRecords (it doesn't know how to map them).

The former change is needed because if we load from an old debug format
bitcode file but load directly into the new format _and then convert
back to the old mode after_, the use-lists of the debug intrinsic
functions (the functions' global value uses) change.