swift234

Enforce maximum allowed access for extensions with 'where' clauses.

Otherwise, we end up with declarations with public access that do not
have public 'self' types. These declarations can then be used by other
modules, which may end up trying to access non-external symbols.

This closes a loophole currently in use by the standard library, so
the '_prext_ReverseIndexType' and '_ReverseCollectionType' protocols
become public for now. In order to keep the API impact minimized,
extensions involving these protocols now extend them directly, so that
all of the "private" stuff shows up in one place in the generated
interface. This is not a long-term solution, but it's no worse than
the rest of the underscore rules in the standard library.

rdar://problem/21380336 tracks relaxing access restrictions for protocol
conformances when the witnesses come from a different type, like a
protocol extension. This requires some SILGen work to do correctly.

Finishes rdar://problem/21559986

Swift SVN r30612