Start handling stack depth issues in explorer (#2799)

Adds stack space handling similar to Clang's approach, but with more support for forwarding return values. Refactors ParseAndExecute for better sharing, and for centralization of the InitStackSpace handling.

Fixes #2795

explorer/interpreter/stack_space.cpp

@@ -0,0 +1,44 @@
+// 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
+
+#include "explorer/interpreter/stack_space.h"
+
+#include "common/check.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/CrashRecoveryContext.h"
+
+namespace Carbon::Internal {
+
+static constexpr int64_t SufficientStack = 256 << 10;
+static constexpr int64_t DesiredStackSpace = 8 << 20;
+
+static LLVM_THREAD_LOCAL intptr_t bottom_of_stack = 0;
+
+// Returns the current bottom of stack.
+static auto GetStackPointer() -> intptr_t {
+  char char_on_stack = 0;
+  char* volatile ptr = &char_on_stack;
+  return reinterpret_cast<intptr_t>(ptr);
+}
+
+auto IsStackSpaceNearlyExhausted() -> bool {
+  if (bottom_of_stack == 0) {
+    // Not initialized on the thread; always start a new thread.
+    return true;
+  }
+  return std::abs(GetStackPointer() - bottom_of_stack) >
+         (DesiredStackSpace - SufficientStack);
+}
+
+auto RunWithExtraStackHelper(llvm::function_ref<void()> fn) -> void {
+  llvm::CrashRecoveryContext context;
+  context.RunSafelyOnThread(
+      [&] {
+        bottom_of_stack = GetStackPointer();
+        fn();
+      },
+      DesiredStackSpace);
+}
+
+}  // namespace Carbon::Internal