[APINotes] Add a 'SwiftImportAsAccessors' entry for properties. (#40)

If true, Swift will import the property as a getter and setter (or
just a getter for read-only properties), rather than as a Swift 'var'
property. At least, it will once this lands and the Swift side gets
implemented.

Part of rdar://problem/28455962
(cherry picked from commit ac91fd5da8be22915b9bf9592e20536df6a18b70)
11 files changed