Use BIR Drop analysis in backend cleanup

Objective

Connect the straight-line BIR Drop result to backend cleanup. Skip backend Drop calls classified as Dead.

Why it was needed

PR #4730 could classify the source local in a move as Dead, but backend cleanup did not use that result yet.

The backend could still call Drop on the moved-from local.

This PR connected the analysis to cleanup, skipped the dead Drop call, and kept the Drop call for the destination.

Main changes

• Records the HirId of each whole local classified as definitely dead.
• Clears old Drop-analysis results before analyzing each BIR function.
• Skips definitely-dead candidates when backend cleanup is built.

Test case

• drop-whole-local-move.rs: Checks that a moved value is dropped through its destination while the moved-from local is skipped. A normal initialized local is still dropped.

Scope of this PR

The backend connection is limited to whole variables.

Projections and partial moves are not supported by this patch.