swift234

Add a new OptionalEnum type to encapsulate the "enum-value-or-nothing" pattern.

This implements the logic of storing "absent" values as 0 and "present"
values as the enumerator's underlying value plus 1. For enums whose raw
values are arbitrary or small, this is a safe, space-efficient way to add
the "absent" value.

The type has been designed to be used with PointerIntPair, so it converts
(explicitly) to intptr_t, and (explicitly) from any integral type.

Adopt this for use with the Accessibility bits in a ValueDecl.

No intended functionality change.

Swift SVN r20597