When the outermost property wrapper associated with a property has a
`wrapperValue`, create the projection property (with the `$` prefix)
at the same access level as the original property. This puts the
wrapped-value interface and the projection interface at the same level.
The newly-introduced @_projectionValueProperty attribute is implicitly
created to establish the link between the original property and the
projection value within module interfaces, where both properties will
be explicitly written out.
The merge-modules phase doesn't have an active type checker, so we were bailing
out of property-wrapper queries before we had a chance to check the cache.
Move the check later, to the points where we actually need a type checker.
Fixes SR-10844 / rdar://problem/51484958.
