Explorer: Fuzzer issue around infinite recursion (#2741)

I limited the stack size to 1K we can change it or make it dynamic from CLI later

Closes #2733

explorer/interpreter/action_stack.h

@@ -119,6 +119,8 @@ class ActionStack {
 
   void Pop() { todo_.Pop(); }
 
+  auto Count() const -> int { return todo_.Count(); }
+
  private:
   // Pop any ScopeActions from the top of the stack, propagating results as
   // needed, to restore the invariant that todo_.Top() is not a ScopeAction.

explorer/interpreter/interpreter.cpp

@@ -952,6 +952,10 @@ auto Interpreter::CallFunction(const CallExpression& call,
                                Nonnull<const Value*> fun,
                                Nonnull<const Value*> arg,
                                ImplWitnessMap&& witnesses) -> ErrorOr<Success> {
+  constexpr int StackSizeLimit = 1000;
+  if (todo_.Count() > StackSizeLimit) {
+    return ProgramError(call.source_loc()) << "stack overflow";
+  }
   if (trace_stream_->is_enabled()) {
     *trace_stream_ << "calling function: " << *fun << "\n";
   }

explorer/testdata/function/fail_recursion_stackoverflow.carbon

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package EmptyIdentifier impl;
+
+fn A() {
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/function/fail_recursion_stackoverflow.carbon:[[@LINE+1]]: stack overflow
+  A();
+}
+
+fn Main() -> i32
+{
+  A();
+  return 0;
+}