[pseudo] NFC, bail out the parse loop if there is no active heads.

[hokein]

Summary

And print the line number in the debug message, which makes it easier to
locate the position where we fail to parse.

clang-tools-extra/pseudo/lib/GLR.cpp

for (const auto &Action : Params.Table.getActions(Head->State, NextTok)) {
         llvm_unreachable("unexpected action kind!");
       }
     }
   };
   std::vector<const GSS::Node *> NewHeads = {
       GSS.addNode(/*State=*/Params.Table.getStartState(Target),
                   /*ForestNode=*/nullptr, {})};
   for (const ForestNode &Terminal : Terminals) {
-    LLVM_DEBUG(llvm::dbgs() << llvm::formatv("Next token {0} (id={1})\n",
-                                             G.symbolName(Terminal.symbol()),
-                                             Terminal.symbol()));
+    LLVM_DEBUG(llvm::dbgs() << llvm::formatv(
+                   "Next token {0} (id={1}, line={2})\n",
+                   G.symbolName(Terminal.symbol()), Terminal.symbol(),
+                   Tokens.tokens()[Terminal.startTokenIndex()].Line + 1));
+    // No active heads, we fail to parse the input, bail out.
+    if (NewHeads.empty())
+      break;
+
     for (const auto *Head : NewHeads)
       AddSteps(Head, Terminal.symbol());
     NewHeads.clear();
     glrReduce(PendingReduce, Params,
               [&](const GSS::Node * NewHead) {
                 // A reduce will enable more steps.
                 AddSteps(NewHead, Terminal.symbol());
               });