One minor revision: this lifts the proposed restriction against
overriding a non-open method with an open one. On reflection,
that was inconsistent with the existing rule permitting non-public
methods to be overridden with public ones. The restriction on
subclassing a non-open class with an open class remains, and is
in fact consistent with the existing access rule.
SR-1129 tracks an issue in which the linking step fails if a base class,
defined in an imported module with a `public private(set) var`, is subclassed
by another module. This adds a test that XFAILs on Linux. The test
passed on OS X.
This issue is currently forcing swift-corelibs-xctest and
swift-corelibs-foundation to mark several privately-used variables as
`public`.
