`DefIndex` can use `newtype_index!`

[petrochenkov]

#60647 (comment)

Note, this could probably be further simplified by using newtype_index! for DefIndex and switching things to IndexVec<DefIndex, _> where appropriate. But let's land this first :)

[petrochenkov]

cc @fabric-and-ink

I will look into it!

[petrochenkov]

Two more possible cleanups that are possible after removal of DefIndexAddressSpace:
#60647 (comment)
#60647 (comment)

I think, this can be closed now.

[oli-obk]

@fabric-and-ink the two linked comments are still unresolved I think?

You are right. I had the original topic in mind.

One of the linked comments is resolved. I don't know how to resolve the second one.

[petrochenkov]

The second is fixed in #59953, closing this issue.