Consider changing "mono item" terminology

[davidtwco]

Requested by @eddyb in this comment. After polymorphisation (#69749) lands, mono items may not actually be monomorphic and so it might be worth changing the terminology here, e.g. "codegen item".

[fmease]

#133883 removed polymorphization, all of its tests and crash tests.

Given the MCP criticized the way the now removed implementation of polymorphization invalidated a lot of assumptions when optimizing MIR and when monomorphizing, I presume that any future design will work rather differently and thus this issue won't apply directly anymore. Feel free to correct me.

Closing as not planned.