[Support] Make error banner optional in logAllUnhandledErrors

In a lot of places an empty string was passed as the ErrorBanner to
logAllUnhandledErrors. This patch makes that argument optional to
simplify the call sites.

llvm-svn: 346604
diff --git a/llvm/tools/llvm-readobj/llvm-readobj.h b/llvm/tools/llvm-readobj/llvm-readobj.h
index 374ffd0..73eee76 100644
--- a/llvm/tools/llvm-readobj/llvm-readobj.h
+++ b/llvm/tools/llvm-readobj/llvm-readobj.h
@@ -40,7 +40,7 @@
       return *EO;
     std::string Buf;
     raw_string_ostream OS(Buf);
-    logAllUnhandledErrors(EO.takeError(), OS, "");
+    logAllUnhandledErrors(EO.takeError(), OS);
     OS.flush();
     reportError(Buf);
   }