Fix crash when using keypaths on final classes in Predicates

[jmschonfeld]

This predicate code validates that the provided KeyPath only contains a single component. Originally, for stored property components, this code checked the payload value to see if it was an inline offset or the out-of-line sentinel defined by the KeyPath ABI documentation to determine if the buffer size matches the expected single component buffer size. However, this document was determined to be incorrect and there are actually multiple sentinel values defined. #216 attempted to correct this by switching to MemoryLayout.offset(of:) to check for a single stored property component. However, MemoryLayout.offset(of:) is also not an appropriate way to check for a single stored property component as not all single stored components have offsets (for example, a property in a final class).

This changes the implementation back to the original style of check before #216 but with a corrected check for whether the offset is stored inline (based on the existence of multiple sentinel values and the max inline offset defined here). This fixes the regression caused for final class properties while still ensuring that we respect other sentinel values as well.

Resolves #1173

[jmschonfeld]

@swift-ci please test