Support function-scope Drops on normal function exit

Objective

Add function-scope Drop handling for normal exits.

Cover unit tail expressions and function parameters.

Why it was needed

A function can end with a tail expression of type (). A call to another unit-returning function is one example.

gccrs did not emit local Drop calls after this form, and it did not register droppable parameters.

This PR added both cases to normal function-exit cleanup.

Main changes

• Emits the current scope’s Drop calls after compiling a unit tail expression.
• Collects droppable parameters while compiling the parameter list, then registers them after creating the function-body block.
• This registration order makes body locals drop before parameters on a normal function exit.

Test cases

• drop-function-scope-unit-tail.rs: Runs one function with a unit call and one with () as the tail expression; both locals are dropped.
• drop-function-params.rs: Covers named and wildcard parameters. The expected output checks that each body local drops before its parameter.

Later change

At this point, parameters and body locals still shared one Drop-candidate list.

PR #4718 later separated their scopes.