optionalRequirementOf relationships take precedence over non-optional ones

[QuietMisdreavus]

Bug/issue #, if applicable: rdar://155967920

Summary

Prior to Swift 6.1 (specifically swiftlang/swift#76983), Swift symbol graphs would contain both requirementOf and optionalRequirementOf relationships for imported optional Objective-C protocol methods. This can confuse Swift-DocC into nondeterministically displaying a "Required" marker for optional requirements based on which relationship it reads last. This PR addresses this nondeterminism by making optionalRequirementOf relationships take precedence over requirementOf ones in the SymbolGraphRelationshipsBuilder.

Dependencies

None

Testing

Steps:

1. Render documentation for a module with an imported Objective-C optional protocol requirement, as mentioned above.
2. Ensure that the "Required" indicator is not rendered for the optional requirement, even on repeated runs.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• [ n/a ] Updated documentation if necessary

[QuietMisdreavus]

@swift-ci Please test

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@swift-ci Please test macOS