Fixing the polly build.

I broke the polly build in r238505. This fixes the failure by adding non-const iterator erase methods to cl::list_storage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238509 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/CommandLine.h

@@ -1323,6 +1323,11 @@ public:
     return Storage.erase(first, last);
   }
 
+  iterator erase(iterator pos) { return Storage.erase(pos); }
+  iterator erase(iterator first, iterator last) {
+    return Storage.erase(first, last);
+  }
+
   iterator insert(const_iterator pos, const DataType &value) {
     return Storage.insert(pos, value);
   }