[SR-12836] Extensions with access modifiers don't have such accesses indexed

[swift-ci]

Previous ID	SR-12836
Radar	None
Original Reporter	rockbruno (JIRA User)
Type	Bug

Environment

Xcode 11.4.1

Additional Detail from JIRA

Votes	0
Component/s	Source Tooling
Labels	Bug
Assignee	None
Priority	Medium

md5: 26ee7c5b26ba72044d15e3771f8c476d

Issue Description:

Here, SourceKit will correctly mark `foo` with the `public` attribute when indexing:

extension Int {
    public func foo() {}
}

If you place the access modifier in the extension, however:

public extension Int {
    func foo() {}
}

...neither the extension or `foo` will have `public` as an access attribute in the result of a index.source SourceKit request.

[swift-ci]

Comment by Bruno Rocha (JIRA)

As explained by @xwu in #31942 SourceKit isn't wrong here. We could have an attribute that represents the `effective` access level instead if doing so is a good idea.