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/compiler/parser.ts

@@ -607,7 +607,7 @@ namespace ts {
     }
 
     export function createSourceFile(fileName: string, sourceText: string, languageVersion: ScriptTarget, setParentNodes = false, scriptKind?: ScriptKind): SourceFile {
-        tracing.push(tracing.Phase.Parse, "createSourceFile", { path: fileName }, /*separateBeginAndEnd*/ true);
+        tracing?.push(tracing.Phase.Parse, "createSourceFile", { path: fileName }, /*separateBeginAndEnd*/ true);
         performance.mark("beforeParse");
         let result: SourceFile;
 
@@ -622,7 +622,7 @@ namespace ts {
 
         performance.mark("afterParse");
         performance.measure("Parse", "beforeParse", "afterParse");
-        tracing.pop();
+        tracing?.pop();
         return result;
     }