Evaluate non-unit tail expressions before Drops

Objective

Evaluate a non-unit tail expression before local Drop calls.

Save the result, run cleanup, and then return the saved value.

Why it was needed

A non-unit tail expression supplies the function’s return value. It must run before local variables are dropped.

The old lowering ran cleanup first, so a tail call could happen too late.

This PR saves the value before cleanup and returns it afterward.

Main changes

• Stores the tail-expression result in the function return slot before emitting Drop calls.
• Runs the local Drop calls.
• Returns the saved value from the function return slot after the Drop calls.

Test case

• drop-function-scope-non-unit-tail.rs: Expected output f then d checks that the tail call runs before the local Drop.