Auto merge of #115138 - cjgillot:dse-move-packed, r=compiler-errors

Do not convert copies of packed projections to moves.

This code path was introduced in rust-lang/rust#113758

After seeing https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Packed.20fields.20and.20in-place.20function.20argument.2Freturn.20passing, this may be UB, so should be disallowed.

This should not appear in normally-built MIR, which introduces temporary copies for packed projections.

Author: bors