[AST] Mark init accessors as having internal access

This allows them to be used across files in the same module
and requires explicit `@usableFromInline` annotation to be
used inside of `@_alwaysEmitIntoClient` and `@inlinable`
initializers.

(cherry picked from commit dbe151b)

Author: xedin

lib/AST/AccessRequests.cpp

@@ -65,8 +65,8 @@ AccessLevelRequest::evaluate(Evaluator &evaluator, ValueDecl *D) const {
       // These are only needed to synthesize the setter.
       return AccessLevel::Private;
     case AccessorKind::Init:
-      // These are only called from designated initializers.
-      return AccessLevel::Private;
+      // These are only called from within the same module.
+      return AccessLevel::Internal;
     }
   }