Auto merge of #143376 - dianne:guard-scope, r=matthewjasper

add a scope for `if let` guard temporaries and bindings

This fixes my concern with `if let` guard drop order, namely that the guard's bindings and temporaries were being dropped after their arm's pattern's bindings, instead of before (https://github.com/rust-lang/rust/pull/141295#issuecomment-2968975596). The guard's bindings and temporaries now live in a new scope, which extends until (but not past) the end of the arm, guaranteeing they're dropped before the arm's pattern's bindings.

This only introduces a new scope for match arms with guards. Perf results (https://github.com/rust-lang/rust/pull/143376#issuecomment-3034922617) seemed to indicate there wasn't a significant hit to introduce a new scope on all match arms, but guard patterns (rust-lang/rust#129967) will likely benefit from only adding new scopes when necessary (with some patterns requiring multiple nested scopes).

Tracking issue for `if_let_guard`: rust-lang/rust#51114

Tests are adapted from examples by `@traviscross,` `@est31,` and myself on rust-lang/rust#141295.

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