llvm-ar: Don't try to extract from thin archives.

This matches the gnu ar behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242162 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Object/Archive.h

@@ -183,6 +183,7 @@ public:
   };
 
   Kind kind() const { return (Kind)Format; }
+  bool isThin() const { return IsThin; }
 
   child_iterator child_begin(bool SkipInternal = true) const;
   child_iterator child_end() const;