[diagtool] Use StringRef::consume_front (NFC)

Author: kazutakahirata

clang/tools/diagtool/TreeView.cpp

@@ -160,8 +160,7 @@ int TreeView::run(unsigned int argc, char **argv, llvm::raw_ostream &out) {
     break;
   case 1:
     RootGroup = argv[0];
-    if (RootGroup.starts_with("-W"))
-      RootGroup = RootGroup.substr(2);
+    RootGroup.consume_front("-W");
     if (RootGroup == "everything")
       ShowAll = true;
     // FIXME: Handle other special warning flags, like -pedantic.