[Runtime] Change the unowned reference fatal error to "was already destroyed."

The "was already deallocated" message is incorrect, since the target of an unowned reference stays allocated even after being deinitialized. We could say "was already deinitialized" but that's a bit of a niche term. "Was already destroyed" conveys what happened without the reader needing to worry about deinitialization versus deallocation.

rdar://149237704

Author: mikeash

stdlib/public/runtime/Errors.cpp

@@ -506,11 +506,11 @@ void swift::swift_abortRetainUnowned(const void *object) {
   if (object) {
     swift::fatalError(FatalErrorFlags::ReportBacktrace,
                       "Fatal error: Attempted to read an unowned reference but "
-                      "object %p was already deallocated\n", object);
+                      "object %p was already destroyed\n", object);
   } else {
     swift::fatalError(FatalErrorFlags::ReportBacktrace,
                       "Fatal error: Attempted to read an unowned reference but "
-                      "the object was already deallocated\n");
+                      "the object was already destroyed\n");
   }
 }