Deduplicate linker inputs in merged linking contexts

[aaronsky]

• adds an example of an app with an extension that both link a shared framework consuming Swift code. repros Linker inputs are not deduplicated in Bazel 8, causing linker warnings around duplicate rpaths #2687.
• introduces a fix wherein entire linker inputs are deduplicated upon subtracting and combining into one linking context. This primarily affects duplicate linker inputs from SwiftToolchainInfo, but could have other effects.
  • This has the potential to affect linker param order, which is meaningful. Reviewers may want to consider this a breaking change.

I would like any reviewers to test the patch out in their own repo before approving, using either Bazel 7 or 8, to help ensure minimal breakage.

[aaronsky]

I would feel better if CI was passing before landing this. As mentioned above, I need to do more testing of this version of the patch internally as well, once I'm back from vacation.

[luispadron]

This fixed our duplicate link warnings for all our top-level targets! It would be nice to be able to test the link flags for our test cases, but we can add that after