Move tracing_flag into a common directory (#669)

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

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