Implement `-check=nullderef=on`? (D v2.113)

[kinke]

This feature was implemented in DMD's glue layer, and would need to be duplicated (or moved to the frontend upstream instead?). In #5113, I've only added ldmd2 driver support for -check=nullderef=off, bailing out when trying to enable it.

[JohanEngelen]

FWIW, the dmd implementation is on machine code level, which is useless in my opinion. The null deref should be a language level check, i.e. in frontend. See here a trivial example where it does not deliver: https://d.godbolt.org/z/sn3eKEzrv .

Edit: no longer sure about frontend/backend (no exactly clear where the division is), but for example this check should happen before devirtualization.