[QoI] Improve diagnostics for unbound generic types

[rintaro]

Since the subject type can be declared in other modules, we should use the Decl itself instead of its location.

On:

let a : Array

Previously:

test.swift:1:9: error: reference to generic type 'Array' requires arguments in <...>
let a : Array
        ^
             <Any>
<unknown>:0: note: generic type 'Array' declared here

Now:

test.swift:1:9: error: reference to generic type 'Array' requires arguments in <...>
let a : Array
        ^
             <Any>
Swift.Array:281:15: note: generic type 'Array' declared here
public struct Array<Element> : RandomAccessCollection, MutableCollection, _DestructorSafeContainer {
              ^

[rintaro]

How can we test this? 🤔

[rintaro]

@swift-ci Please smoke test

[rintaro]

Waiting for #7197 to be approved.

[rintaro]

@swift-ci Please smoke test