Extension Block Symbols: Fix inheritsFrom Relationship

[theMomax]

This PR fixes a bug in the extension block format for Symbol Graphs. When extending an external swift.class symbol that has a superclass, the respective inheritsFrom relationship between the extended swift.class symbol and its superclass symbol was emitted not only in the symbol graph file that includes the external class' symbol, but also in every symbol graph file that contains a swift.extension symbol extending the same swift.class symbol.

E.g. in the following snippet both External.symbols.json as well as Main@External.symbols.json included the inheritsFrom relationship between C and Superclass.

// "External" Target
public class Superclass { }
public class C: Superclass { }

// "Main" Target
public extension C {
    func foo() { }
}

This bug has no associated issue or radar.

[jaedon6]

waiting for checks to complete

[QuietMisdreavus]

This looks good, thanks!

[QuietMisdreavus]

@swift-ci Please smoke test

[theMomax]

@QuietMisdreavus thanks! This can be merged right away independently of the other PRs.