Update code to compile without issue when ExistentialAny enabled

[pseligman]

Motivation:

We should feel confident that we compile clean when built in an environment that has enabled upcoming Swift features.

When compiling swift-asn1 specifying .enableUpcomingFeature("ExistentialAny") there are some minor warnings emitted by ASN1.swift.

Modifications:

Minor code changes to address surfaced warnings

Result:

Better confidence that we don't break downstreams that compile this code with these upcoming features enabled.

[Lukasa]

@swift-server-bot add to allowlist