add command-line flag to enable/disable tracing output (#325)

* add command-line flag to enable/disable tracing output

* adding missing exit for pattern variable in wrong context and a test case for it (#324)

* Update executable_semantics/interpreter/interpreter.cpp

comment on separate line as code

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* fix interpreter's handling of optional else of if statement (#323)

* Update pattern_variable_fail.golden due to error (#334)

* Use llvm's CommandLine for parsing (#332)

* GitHub testing action (#331)

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

* add copyright

* Create a Dictionary abstraction over the raw Cons list. (#327)

* Create a Dictionary abstraction over the raw Cons list.

* renamed Cons and some methods of Dictionary, various other cleanup

* Update executable_semantics/tracing_flag.cpp

added namespace comment

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* added a comment to cpp file

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>
Co-authored-by: Dave Abrahams <dabrahams@google.com>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

executable_semantics/main.cpp

@@ -7,6 +7,7 @@
 #include <iostream>
 
 #include "executable_semantics/syntax_helpers.h"
+#include "executable_semantics/tracing_flag.h"
 #include "llvm/Support/CommandLine.h"
 
 extern FILE* yyin;
@@ -17,8 +18,7 @@ int main(int argc, char* argv[]) {
 
   using llvm::cl::desc;
   using llvm::cl::opt;
-  // TODO: Add with related support.
-  // opt<bool> quiet_option("quiet", desc("Disable tracing"));
+  opt<bool> quiet_option("quiet", desc("Disable tracing"));
   opt<std::string> input_filename(llvm::cl::Positional, desc("<input file>"));
   llvm::cl::ParseCommandLineOptions(argc, argv);
 
@@ -31,5 +31,8 @@ int main(int argc, char* argv[]) {
       return 1;
     }
   }
+  if (quiet_option) {
+    Carbon::tracing_output = false;
+  }
   return yyparse();
 }