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

include/swift/Runtime/HeapObject.h

@@ -169,6 +169,10 @@ SWIFT_RUNTIME_EXPORT
 BoxPair::Return swift_makeBoxUnique(OpaqueValue *buffer, Metadata const *type,
                                     size_t alignMask);
 
+/// Returns the address of a heap object representing all empty box types.
+SWIFT_RUNTIME_EXPORT
+HeapObject* swift_allocEmptyBox();
+
 // Allocate plain old memory. This is the generalized entry point
 // Never returns nil. The returned memory is uninitialized. 
 //

include/swift/Runtime/Metadata.h

@@ -1761,6 +1761,9 @@ struct TargetHeapLocalVariableMetadata
   static bool classof(const TargetMetadata<Runtime> *metadata) {
     return metadata->getKind() == MetadataKind::HeapLocalVariable;
   }
+  constexpr TargetHeapLocalVariableMetadata()
+      : TargetHeapMetadata<Runtime>(MetadataKind::HeapLocalVariable),
+        OffsetToFirstCapture(0), CaptureDescription(nullptr) {}
 };
 using HeapLocalVariableMetadata
   = TargetHeapLocalVariableMetadata<InProcess>;

include/swift/Runtime/RuntimeFunctions.def

@@ -73,6 +73,11 @@ FUNCTION(ProjectBox, swift_projectBox, DefaultCC,
          ARGS(RefCountedPtrTy),
          ATTRS(NoUnwind, ReadNone))
 
+FUNCTION(AllocEmptyBox, swift_allocEmptyBox, DefaultCC,
+         RETURNS(RefCountedPtrTy),
+         ARGS(),
+         ATTRS(NoUnwind))
+
 // RefCounted *swift_allocObject(Metadata *type, size_t size, size_t alignMask);
 FUNCTION_WITH_GLOBAL_SYMBOL_AND_IMPL(AllocObject, swift_allocObject,
          _swift_allocObject, _swift_allocObject_, RegisterPreservingCC,