[5.5][CodeCompletion] Revert migration of key path completion to solver-based

[ahoppen]

• Explanation: [5.5][CodeCompletion] Migrate key path completion to be solver based #38178 migrated key-path completion to the new solver-based approach, looking to improve code completion of key paths in generic contexts. When the stress tester started running again, it found some issues, which were fixed by [5.5][Sema] Fix two assertion failures/crashes in solver-based key-path completion #38300. Recently, the stress tester found more another crash around key path completion and I judge the risk of the change too be too for 5.5 now.
• Scope: Code completion inside key paths
• Risk: Low (this restores the old key path completion behavior with known missing completions but no known crashes)
• Testing: Swift CI is running the test suite before the PRs
• Issue: rdar://80474233
• Reviewer: @xedin (Pavel Yaskevich)

[ahoppen]

@swift-ci Please test