Revert [AST] Mark @completionHandlerAsync user-inaccessible

This is too extreme, as we still want the attribute
to appear in things like code completion and
generated interfaces if it was explicitly written
by the user.

Reverts a2b5f68,
but keeps the test case intact.

Author: hamishknight

include/swift/AST/Attr.def

@@ -625,7 +625,7 @@ SIMPLE_DECL_ATTR(reasync, AtReasync,
   110)
 
 DECL_ATTR(completionHandlerAsync, CompletionHandlerAsync,
-  OnAbstractFunction | ConcurrencyOnly | LongAttribute | UserInaccessible |
+  OnAbstractFunction | ConcurrencyOnly | LongAttribute |
   ABIStableToAdd | ABIStableToRemove |
   APIStableToAdd | APIStableToRemove,
   111)