Regardless of the value specified for `-unavailable-decl-optimization`, decls
that are unavailable in custom availability domains should be treated as always
unreachable at runtime.
Part of rdar://138441307.
It should no longer be necessary to provide an `@_alwaysEmitIntoClient` version
of `_diagnoseUnavailableCodeReached()`. This workaround was originally added to
provide compatibility with projects that were misconfigured to compile against
a newer stdlib but link against an older one.
Resolves rdar://119892482.
The declarations in a zippered macOS library may be referenced by clients that
build either for macOS or for macCatalyst. It is therefore inappropriate to
stub functions in zippered libraries as "unreachable" when they are only
unavailable on macOS. Making this logic correct is a larger project, so for now
just disable stubbing when there is a target variant.
Resolves rdar://125371621
