Fix a warning about casting a const pointer to void*

[n.bozhenov]

Summary

rL314336 introduced a new compile time warning about casting a const pointer to void*. Because of that the compiler fails to be built with -Werror flag. The suggested patch fixes the issue.

[n.bozhenov]

Seems that it has been just fixed by rL314424 :-)
Abandoning this review.

clang/lib/AST/ExternalASTMerger.cpp

for (const std::unique_ptr<ASTImporter> &Importer : Importers) {
       if (auto SourceDC = LookupSameContext(SourceTU, DC, Reverse)) {
         DidCallback = true;
         if (Callback(*Importer, Reverse, SourceDC))
           break;
       }
     }
     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");
   }
 }
 
 void ExternalASTMerger::CompleteType(TagDecl *Tag) {
   assert(Tag->hasExternalLexicalStorage());