Assert that non-extended temporaries and `super let` bindings have scopes
This PR clarifies a point of confusion in the compiler: all bodies have an outer temporary drop scope, including `static` and `const` item bodies[^1]. Whenever a temporary should be dropped in its enclosing temporary scope, it should have a temporary scope to be dropped in so that its drop can be scheduled[^2]. As such, I've updated some relevant comments and made `ScopeTree::default_temporary_scope` and `RvalueScopes::temporary_scope` panic when an enclosing temporary scope isn't found instead of allowing potential bugs where potentially-drop-sensitive temporaries are effectively given static lifetimes.
Since non-extended `super let` bindings are dropped in their block's enclosing temporary scope, this applies to them as well: the enclosing temporary scope should exist.
[^1]: See https://github.com/rust-lang/rust/blob/master/compiler/rustc_hir_analysis/src/check/region.rs#L773-L778 for non-`fn`/closure bodies. The `this.cx.var_parent = None;` enables [lifetime extension to `'static` lifetimes](https://doc.rust-lang.org/stable/reference/destructors.html#r-destructors.scope.lifetime-extension.static) and the `ScopeData::Destruction` scope ensures non-extended temporaries are dropped in the body expression's scope.
[^2]: For certain borrowed temporaries, drops that don't require running destructors may later be removed by constant promotion. That is unrelated to this PR.
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