Re-do tracing initialization and tests around calls

Make `tracing` either `undefined` or the same namespace as before.
Switching all calls to `tracing?.___` means that there is no cost for
a call or the arguments when tracing is not used.  Comparing two runs
without tracing (27 runs, drop 5+5, avg rest) I get:

    master:
      42.59s user 1.00s system 165% cpu 26.372 total
    changed:
      42.01s user 0.982 system 165% cpu 26.039 total

(Makes it all private, so no api changes.)

src/executeCommandLine/executeCommandLine.ts

@@ -501,7 +501,7 @@ namespace ts {
         updateSolutionBuilderHost(sys, cb, buildHost);
         const builder = createSolutionBuilder(buildHost, projects, buildOptions);
         const exitStatus = buildOptions.clean ? builder.clean() : builder.build();
-        tracing.dumpLegend();
+        tracing?.dumpLegend();
         return sys.exit(exitStatus);
     }
 
@@ -666,7 +666,8 @@ namespace ts {
         }
 
         if (canTrace(system, compilerOptions)) {
-            tracing.startTracing(isBuildMode ? tracing.Mode.Build : tracing.Mode.Project, compilerOptions.generateTrace!, compilerOptions.configFilePath);
+            startTracing(isBuildMode ? tracingEnabled.Mode.Build : tracingEnabled.Mode.Project,
+                         compilerOptions.generateTrace!, compilerOptions.configFilePath);
         }
     }
 
@@ -674,7 +675,7 @@ namespace ts {
         const compilerOptions = program.getCompilerOptions();
 
         if (canTrace(sys, compilerOptions)) {
-            tracing.stopTracing(program.getTypeCatalog());
+            tracing?.stopTracing(program.getTypeCatalog());
         }
 
         let statistics: Statistic[];