Handle the error of trying to convert a regular archive to a thin one.

While at it, test that we can add to a thin archive.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242330 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Object/Archive.h

@@ -74,6 +74,7 @@ public:
       return Data.begin() < other.Data.begin();
     }
 
+    const Archive *getParent() const { return Parent; }
     Child getNext() const;
 
     ErrorOr<StringRef> getName() const;