Move tracing_flag into a common directory (#669)

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

executable_semantics/syntax/BUILD

@@ -30,9 +30,9 @@ cc_library(
     deps = [
         ":paren_contents",
         "//common:ostream",
-        "//executable_semantics:tracing_flag",
         "//executable_semantics/ast:declaration",
         "//executable_semantics/ast:expression",
+        "//executable_semantics/common:tracing_flag",
         "//executable_semantics/interpreter",
         "//executable_semantics/interpreter:typecheck",
     ],

executable_semantics/syntax/lexer.lpp

@@ -8,7 +8,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #include <cstdlib>
 #include <iostream>
 
-#include "executable_semantics/tracing_flag.h"
+#include "executable_semantics/common/tracing_flag.h"
 #include "executable_semantics/syntax/parse_and_lex_context.h"
 #include "llvm/ADT/StringExtras.h"
 %}

executable_semantics/syntax/parse.cpp

@@ -6,9 +6,9 @@
 
 #include <iostream>
 
+#include "executable_semantics/common/tracing_flag.h"
 #include "executable_semantics/syntax/parse_and_lex_context.h"
 #include "executable_semantics/syntax/parser.h"
-#include "executable_semantics/tracing_flag.h"
 
 extern FILE* yyin;
 

executable_semantics/syntax/syntax_helpers.cpp

@@ -5,9 +5,9 @@
 #include "executable_semantics/syntax/syntax_helpers.h"
 
 #include "common/ostream.h"
+#include "executable_semantics/common/tracing_flag.h"
 #include "executable_semantics/interpreter/interpreter.h"
 #include "executable_semantics/interpreter/typecheck.h"
-#include "executable_semantics/tracing_flag.h"
 
 namespace Carbon {