IRGen: EmptyBoxType's representation cannot be nil because of a conflict with extra inhabitant assumption in indirect enums (#10326)

* IRGen: EmptyBoxType's representation cannot be nil because of a conflict with extra inhabitant assumption in indirect enums

We map nil to the .None case of Optional. Instead use a singleton object.

SR-5148
rdar://32618580

stdlib/public/stubs/GlobalObjects.cpp

@@ -35,6 +35,10 @@ ClassMetadata CLASS_METADATA_SYM(s27_RawNativeDictionaryStorage);
 // _direct type metadata for Swift._RawNativeSetStorage
 SWIFT_RUNTIME_STDLIB_INTERFACE
 ClassMetadata CLASS_METADATA_SYM(s20_RawNativeSetStorage);
+
+// _direct type metadata for Swift._EmptyBoxStorage
+SWIFT_RUNTIME_STDLIB_INTERFACE
+ClassMetadata CLASS_METADATA_SYM(s16_EmptyBoxStorage);
 } // namespace swift
 
 swift::_SwiftEmptyArrayStorage swift::_swiftEmptyArrayStorage = {
@@ -127,6 +131,18 @@ void swift::_swift_instantiateInertHeapObject(void *address,
   ::new (address) HeapObject{metadata};
 }
 
+swift::HeapLocalVariableMetadata _emptyBoxStorageMetadata;
+
+/// The signleton empty box storage object.
+swift::_SwiftEmptyBoxStorage swift::_EmptyBoxStorage = {
+ // HeapObject header;
+  {
+    &_emptyBoxStorageMetadata,
+    //&swift::CLASS_METADATA_SYM(s16_EmptyBoxStorage), // isa pointer
+  }
+};
+
+
 namespace llvm { namespace hashing { namespace detail {
   // An extern variable expected by LLVM's hashing templates. We don't link any
   // LLVM libs into the runtime, so define this here.