[NFC] Rename swift_runtime_unreachable to swift_unreachable and make it use LLVM's support when available.

include/swift/Basic/Unreachable.h

@@ -0,0 +1,47 @@
+//===--- Unreachable.h - Implements swift_unreachable ---*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 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_UNREACHABLE_H
+#define SWIFT_BASIC_UNREACHABLE_H
+
+#ifdef SWIFT_LLVM_SUPPORT_IS_AVAILABLE
+
+// The implementation when LLVM is available.
+
+#include "llvm/Support/ErrorHandling.h"
+#define swift_unreachable llvm_unreachable
+
+#else
+
+// The implementation when LLVM is not available.
+
+#include <assert.h>
+#include <stdlib.h>
+
+#include "swift/Runtime/Config.h"
+
+SWIFT_RUNTIME_ATTRIBUTE_NORETURN
+inline static void swift_unreachable(const char *msg) {
+  assert(false && msg);
+  (void)msg;
+  abort();
+}
+
+#endif
+
+#endif