[SymbolGraphGen] don't drop underscored protocol implementation symbols

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Resolves rdar://133086270

Consider the following Swift code:

public protocol _HiddenProtocol {
    associatedtype T
}

public class MyClass: _HiddenProtocol {
    public struct T {}
}

When using SymbolGraphGen with the -skip-protocol-implementations flag, the type MyClass.T is hidden from the symbol graph, even though the protocol it's implementing is also hidden. This PR changes the -skip-protocol-implementations code to check whether the protocol being implemented has an underscored name before dropping the implementation symbol from the symbol graph.

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@swift-ci Please smoke test

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@swift-ci Please smoke test