Fix reborrow ICE in MIR place lowering

[P8L1]

Summary

Fixes #156482.

This fixes an internal THIR/MIR contract inconsistency for generic reborrow expressions.

ExprKind::Reborrow is lowered into MIR as Rvalue::Reborrow, but MIR expression categorization treated it as place-like while expr_as_place treated evaluating it as a place as unreachable. That caused the #156482 ICE, where the compiler panicked with entered unreachable code in expr_as_place while building MIR for a generic reborrow expression.

The fix makes the MIR builder contract explicit: a reborrow expression is rvalue-producing. Its source may be a place, but the reborrow expression itself is not an assignable place.

What changed

• Updated MIR expression categorization so ExprKind::Reborrow is categorized as rvalue-producing rather than place-like.
• Updated place lowering so reborrow follows the normal rvalue temporary-materialization path if a place is needed.
• Added a targeted UI regression test covering the exact [ICE]: Reborrow causes entered unreachable code in expr_as_place. #156482 repro.

cc @aapoalas

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[aapoalas]

LGTM, thank you <3

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