add a require utility function

In contrast to `assert` and similar functions, `require` works in no-assert builds the same way as in debug builds.
It can be used to check invariants also in release builds.

Author: eeckstein

include/swift/Basic/Require.h

@@ -0,0 +1,34 @@
+//===--- Require.h ----------------------------------------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+//  This file defines swift_unreachable, which provides the
+//  functionality of llvm_unreachable without necessarily depending on
+//  the LLVM support libraries.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_BASIC_REQUIRE_H
+#define SWIFT_BASIC_REQUIRE_H
+
+#include "llvm/Support/raw_ostream.h"
+
+/// Checks the `condition` and if it's false abort with `message`.
+/// In contrast to `assert` and similar functions, `require` works in
+/// no-assert builds the same way as in debug builds.
+inline void require(bool condition, const char *message) {
+  if (!condition) {
+    llvm::errs() << message << '\n';
+    abort();
+  }
+}
+
+#endif