Move tests from tests/ui/issues/ to appropriate directories

[ujjwalvishwakarma2006]

In this PR, I am moving the following test from tests/ui/issues directory to the appropriate directories, followed by the addition of issue links at the top and reblessing of the stderr files:

• issue-3874.rs ➝ tests/ui/binding/ref-in-let-lhs-in-field.rs
• issue-4972.{rs,stderr} ➝ tests/ui/box/deref-borrowed-trait-object-in-box.{rs,stderr}

A more minimal version for the second test could be, in my opinion:

trait MyTrait {
    fn dummy(&self) {}
}

struct TraitWrapper(Box<dyn MyTrait + 'static>);

fn get_tw_map(tw: &TraitWrapper) -> &dyn MyTrait {
    match *tw {
        TraitWrapper(box ref map) => map,
    }
}

pub fn main() {}

---

Initially, I decided upon the following more descriptive names, but they were way too big:

• issue-3874.rs ➝ tests/ui/binding/ref-pattern-in-let-lhs-in-enum-field.rs
• issue-4972.{rs,stderr} ➝ tests/ui/box/dereferencing-borrowed-trait-object-inside-box.{rs,stderr}

r? Kivooeo
r? Teapot4195

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