Rollup merge of #121256 - Jarcho:visitor2, r=oli-obk

Allow AST and HIR visitors to return `ControlFlow`

Alternative to #108598.

Since rust-lang/libs-team#187 was rejected, this implements our own version of the `Try` trait (`VisitorResult`) and the `try` macro (`try_visit`). Since this change still allows visitors to return `()`, no changes have been made to the existing ones. They can be done in a separate PR.