Do not use desugared local names in tail_expr_drop_order diagnostics

[qaijuang]

tail_expr_drop_order labels values using names from body.var_debug_info.
That works for user-written bindings, but .await lowering also introduces a synthetic __awaitee binding for debuginfo. This can leak into the user-facing lint diagnostic.

This PR filters collected diagnostic names through LocalDecl::from_compiler_desugaring(), so compiler-generated locals fall back to the existing #N temporary labels. This does not change async lowering or the __awaitee debuginfo name itself.

A user-written binding named __awaitee is still treated as user-written and is still shown by name.

Fixes #136206

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @JonathanBrouwer

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