Auto merge of #115705 - cjgillot:const-prop-aggregate, r=oli-obk

Read from non-scalar constants and statics in dataflow const-prop

DataflowConstProp is designed to handle scalar values. When MIR features an assignment from a non-scalar constant, we need to manually decompose it into the custom state space.

This PR tweaks interpreter callbacks to allow reusing `eval_mir_constant` without having a stack frame to get a span from.

r? `@oli-obk`
cc `@jachris`

Author: bors