Be stricter about identifying reference types.

TypeReferenceOwnership here is a bitmap indicating which of
the various ownership modifiers (weak, unowned, unmanaged)
were present on the field declaration.  Making this a bitmap
ensures that we can accurately represent the data that may
have been provided but we have to be careful when we interpret
the result.

This changes the `isWeak`, `isUnowned`, `isUnmanged` checks to
only return true if exactly one bit is set.  Combinations of bits
will not be accepted.

Also, add `isStrong` to check for the default ownership with
no modifiers.

Author: tbkka

stdlib/public/runtime/Private.h

@@ -50,8 +50,10 @@ class TypeReferenceOwnership {
 
 #define REF_STORAGE(Name, ...) \
   void set##Name() { Data |= Name; } \
-  bool is##Name() const { return Data & Name; }
+  bool is##Name() const { return Data == Name; }
 #include "swift/AST/ReferenceStorage.def"
+
+  bool isStrong() const { return Data == 0; }
 };
 
 /// Type information consists of metadata and its ownership info,
@@ -76,9 +78,11 @@ class TypeInfo {
   const Metadata *getMetadata() const { return Response.Value; }
   MetadataResponse getResponse() const { return Response; }
 
-  bool isWeak() const { return ReferenceOwnership.isWeak(); }
-  bool isUnowned() const { return ReferenceOwnership.isUnowned(); }
-  bool isUnmanaged() const { return ReferenceOwnership.isUnmanaged(); }
+#define REF_STORAGE(Name, ...) \
+  bool is##Name() const { return ReferenceOwnership.is##Name(); }
+#include "swift/AST/ReferenceStorage.def"
+
+  bool isStrong() const { return ReferenceOwnership.isStrong(); }
 
   TypeReferenceOwnership getReferenceOwnership() const {
     return ReferenceOwnership;