fix const_item_mutation lint to use needs_drop instead of has_dtor

[Rachit2323]

Description:
The const_item_mutation lint was only checking if the outer type has a direct Drop impl (has_dtor). But if a field inside the type has a Drop impl, that drop logic can also observe the mutation — so the warning should be suppressed in that case too.

This fixes a false positive where the lint would warn on code like:

O.inner.val = 42;

even when Inner has a Drop impl that prints the value — meaning the mutation IS observable and the warning is wrong.

Fix: replace has_dtor check with needs_drop, which checks the whole type including all fields inside it.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @jackh726

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[rabindra789]

Nice fix! While testing this branch, I noticed one behaviour change. on the base commit this lint warns for traits associated consts whose type mentions Self, but on this branch it doesn't. Is that expected, or should this case still lint?