Auto merge of #2940 - saethlin:use-after-free-spans, r=RalfJung

When reporting a heap use-after-free, say where the allocation was allocated and deallocated

This is a partial solution to: rust-lang/miri#2917

Currently in the interpreter, we only have accurate information for where heap allocations are allocated and deallocated (see rust-lang/miri#2940 (comment)). So this just implements support for allocations where the information is already available, and the full support will require more interpreter tweaks.

Author: bors