[6.3] [Index] Relate ObjC optional protocol methods to protocol requirement

[ileitch]

• Explanation: When a class implements an optional ObjC protocol method, the indexer was not creating a RelationOverrideOf relation to the protocol requirement. Use findWitnessedObjCRequirements to find and index these optional requirements.
• Scope: Purely additive to the index store. Can help tools such as Periphery be more accurate.
• Issues: #56327
• Original PRs: #86167
• Risk: Very low, perhaps a slight increase in index store size.
• Testing: Passed all tests across all platforms.
• Reviewers: @hamishknight

[hamishknight]

@swift-ci please test

[ileitch]

@hamishknight Is this good to merge now?

[hamishknight]

Still needs branch manager approval, need to check whether it meets the bar for 6.3 changes at that this point into the release branch

[hamishknight]

@swift-ci please test