Fix check_expr_if to point to a more accurate location of the compilation error in some cases

[sgasho]

closes #146190

The problem linked on the issue above lays on how Expr::If is processed.

Currently, Expr::If is processed by depth-first like approach, which is natural considering that Expr::If is a nested structure.

e.g.

fn main() { let oa = Some(1); let oa2 = Some(1); let v = if let Some(a) = oa { Some(&a) } else if let Some(a) = oa2 { &Some(a) } else { None }; println!("{v:?}"); }

Simplified Expr::If structure for this code is like below.

Expr::If { cond: "if let Some(a) = oa", then: "Some(&a)", else: { Expr::If { cond: "else if let Some(a) = oa2", then: "&Some(a)", else: { cond: "else", then: "None", else: None, } } } } 

Current check_expr_if processes this Expr::If by depth-first-search like approach, that is why "if and else have incompatible types" occurs between else if let Some(a) = oa2 and else, not between if let Some(a) = oa and if let Some(a) = oa2.

New approach on this PR is first flatten the Expr::If nested nodes, then process forward.

I created _if.rs and moved the check_expr_if becase this new approach requires a bit more lines of codes.

I added a test case that guarantees the problem on the linked issue has been resolved.
tests/ui/expr/if/if-multi-else-if-incompatible-types-issue-146190.rs

Discussion on Zulip: https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/Struggling.20to.20fix.20issue.20.23146190.20check_expr_if/with/540960074

[sgasho]

Was "if and else (have incompatible types) " appropriate for this case?
I think this else if branch has expression of type ! and it is compatible for any type. (source: "Expressions of type ! can be coerced into any other type." at https://doc.rust-lang.org/reference/types/never.html ).

I could be wrong but my understanding is showing "if may be missing an else clause" for this case (all if-else-if have compatible types and the only problem is missing else branch, which would be evaluated to ())

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