[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-objcopy/llvm-objcopy.cpp b/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
index 755c786..bc50880 100644
--- a/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
+++ b/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
@@ -66,7 +66,7 @@
   assert(E);
   std::string Buf;
   raw_string_ostream OS(Buf);
-  logAllUnhandledErrors(std::move(E), OS, "");
+  logAllUnhandledErrors(std::move(E), OS);
   OS.flush();
   WithColor::error(errs(), ToolName) << "'" << File << "': " << Buf;
   exit(1);