Fix -Wcast-qual warning after r314336.

llvm-svn: 314424
diff --git a/clang/lib/AST/ExternalASTMerger.cpp b/clang/lib/AST/ExternalASTMerger.cpp
index ec2e303..6b75c51 100644
--- a/clang/lib/AST/ExternalASTMerger.cpp
+++ b/clang/lib/AST/ExternalASTMerger.cpp
@@ -212,7 +212,7 @@
     }
     if (!DidCallback && LoggingEnabled())
       logs() << "(ExternalASTMerger*)" << (void*)this
-             << " asserting for (DeclContext*)" << (void*)DC
+             << " asserting for (DeclContext*)" << (const void*)DC
              << ", (ASTContext*)" << (void*)&Target.AST
              << "\n";
     assert(DidCallback && "Couldn't find a source context matching our DC");