const-eval.const-expr.borrows: mention indirect places

[RalfJung]

Follow-up to #1858.

[RalfJung]

Based on the last commit I added in rust-lang/rust#140942, instead of a list of things we allow, we could consider just saying what is not allowed -- mutable / interior mutable borrows of lifetime-extended places. Maybe that would be better?

[traviscross]

Thanks @RalfJung; @ehuss and talked about this new language. On first pass, it looks pretty good to us. To your question about stating this in terms of what's not allowed, that also had some appeal to us, and we actually think it'd be best in this case to do both, i.e. to state what's allowed, as is done here, and to then also approach the same truth from the other side by stating what's not allowed. We do that in a number of places, and this seems like a good candidate for it, in terms of being as clear as possible.

[RalfJung]

I've given that a shot, please let me know what you think.

I also realized destructors.scope.lifetime-extension.static fails to mention that it applies specifically to the top-level scope of const/static initializers.