Miri can't detect clashing shims between static and function symbols in all cases

[hkalbasi]

https://godbolt.org/z/r1GbWfz9M

This is accepted currently in miri, but environ conflicts with miri static shim so it should be rejected. The problem is that the map of symbol to shim is not unified and there are multiple maps, so in this case environ is not detected as a shim and so the check_shim_symbol_clash logic doesn't kick in.

[RalfJung]

Specifically the problem is that when we are invoking environ as a function, we are looking for conflicting exported statics called environ, but we are not looking for conflicting native Miri static shims called environ. This would be easy to fix, but the other direction is harder since the other direction would require checking whether for a given name there exists a native Miri function shim. I've long wanted to refactor how we do function shims, and that would also enable this.