[6.3] Disable pack specialization

[aidan-hall]

• Explanation: Disable the PackSpecialization pass in the FunctionPass pipeline, and mark the pack_specialization.swift test file, which relies on the pass, as XFAIL
• Scope: This change has minimal impact, since it is just a 2-line change to temporarily disable an optimization and an associated test. It is highly important, since the current PackSpecialization pass causes compiler crashes when building certain projects (see issue).
• Issues: rdar://164515160
• Original PRs: SILOptimizer: Disable PackSpecialization pass until it has been properly fixed. #85491
• Risk: Lost potential performance gains from the pass.
• Testing: Unit tests (see below).
• Reviewers: @eeckstein

[aidan-hall]

@swift-ci smoke test

[aidan-hall]

@swift-ci test

[tbkka]

Thank you!