Auto merge of #148658 - dianne:cleanup-rvalue-scopes, r=cjgillot

cleanup: merge `RvalueScopes` into `ScopeTree`

This gets rid of `RvalueCandidate`, inlines the definition of `RvalueScopes` into `ScopeTree`, and removes two `RvalueScopes`-specific modules, consolidating the scoping logic a bit. Removing the extra step of going from `RvalueCandidate`s to `RvalueScopes` and removing the duplication between them should also hopefully improve perf.

I've also taken the liberty of doing a bit of renaming and comment updates, changing some "rvalue scope"s to "extended temporary scope"s. This is a bit closer to the Reference's terminology and makes it clearer that it's specific to temporary lifetime extension. This isn't comprehensive. In particular, I've left `record_rvalue_scope_if_borrow_expr` untouched since rust-lang/rust#146098 gets rid of it.

Pulled out from rust-lang/rust#146098.

r? BoxyUwU as the reviewer of rust-lang/rust#146098 (though feel free to reassign/claim! this is just cleanup)

cc `@dingxiangfei2009`

Author: bors