[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-cov/TestingSupport.cpp b/llvm/tools/llvm-cov/TestingSupport.cpp
index e07abdb..16a1c26 100644
--- a/llvm/tools/llvm-cov/TestingSupport.cpp
+++ b/llvm/tools/llvm-cov/TestingSupport.cpp
@@ -33,7 +33,7 @@
   if (!ObjErr) {
     std::string Buf;
     raw_string_ostream OS(Buf);
-    logAllUnhandledErrors(ObjErr.takeError(), OS, "");
+    logAllUnhandledErrors(ObjErr.takeError(), OS);
     OS.flush();
     errs() << "error: " << Buf;
     return 1;