[Index][SR-12903] Index @objc optional references

[rockbruno]

Declarations of objc optional methods were indexed, but their references weren't.

import Foundation

@objc protocol Foo {
    @objc optional func bar()
}

class Sample {

    weak var delegate: Foo?

    func doSomething() {
        delegate?.bar()
    }
}

The bar() call was left unindexed. This provides a fix by walking DynamicMemberRefExprs.

[nathawes]

@swift-ci please test

[nathawes]

LGTM. Thanks for the fix!

[amolpict]

@nathawas Are you people planning to make this fix available in upcoming Swift version release ?