fix: Apply IndexMut obligations for non-assigning mutable index usages

[ShoyuVanilla]

Fixes #17738

Currently, we are pushing IndexMut obligations only for assign usages;

rust-analyzer/crates/hir-ty/src/infer/expr.rs

Lines 809 to 817 in f982f3f

	if *is_assignee_expr {
	if let Some(index_trait) = self.resolve_lang_trait(LangItem::IndexMut) {
	let trait_ref = TyBuilder::trait_ref(self.db, index_trait)
	.push(self_ty)
	.fill(|_| index_ty.clone().cast(Interner))
	.build();
	self.push_obligation(trait_ref.cast(Interner));
	}
	}

[davidbarsky]

LGTM!

[lnicola]

@bors r=davidbarsky

[bors]

📌 Commit b5494d7 has been approved by davidbarsky

It is now in the queue for this repository.

[bors]

⌛ Testing commit b5494d7 with merge c8e41d9...

[bors]

☀️ Test successful - checks-actions
Approved by: davidbarsky
Pushing c8e41d9 to master...