Move tracing_flag into a common directory (#669)

Intended to combine well with #668, so neither is alone.

executable_semantics/common/BUILD

@@ -0,0 +1,12 @@
+# 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
+
+package(default_visibility = ["//executable_semantics:__subpackages__"])
+
+cc_library(
+    name = "tracing_flag",
+    srcs = ["tracing_flag.cpp"],
+    hdrs = ["tracing_flag.h"],
+    visibility = ["//executable_semantics:__subpackages__"],
+)

executable_semantics/common/tracing_flag.cpp

@@ -0,0 +1,11 @@
+// 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 "executable_semantics/common/tracing_flag.h"
+
+namespace Carbon {
+
+bool tracing_output = false;
+
+}  // namespace Carbon

executable_semantics/common/tracing_flag.h

@@ -0,0 +1,15 @@
+// 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
+
+#ifndef EXECUTABLE_SEMANTICS_COMMON_TRACING_FLAG_H_
+#define EXECUTABLE_SEMANTICS_COMMON_TRACING_FLAG_H_
+
+namespace Carbon {
+
+// Program option to enable/disable tracing.
+extern bool tracing_output;
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_COMMON_TRACING_FLAG_H_