[6.0 cherry-pick][KeyPath] Fix regression in == on keypaths

[kubamracek]

• Explanation: A recent change in KeyPath implementation ([embedded] Compile-time (literal) KeyPaths for Embedded Swift #72472) that was done for Embedded Swift and was supposed to be an NFC for regular Swift accidentally commented out an important check inside the implementation of == in AnyKeyPath. This caused a regressions in the equality of keypaths and they were misreporting being equal in some cases when they're not supposed to. [KeyPath] Fix regression in == on keypaths #73347
• Scope: Code that uses KeyPaths and specifically comparing them for equality.
• Risk: Low -- a removal of the accidental changes that reverts back to the implementation of == that has been in use for years.
• Testing: Added test case.
• Issue: rdar://127296261
• Reviewer: @Catfish-Man, @jckarter, @Azoy on [KeyPath] Fix regression in == on keypaths #73347

[kubamracek]

@swift-ci please test