Add -print-zero-stats option and use it in ensure_no_astgen.swift

to verify ExportedSourceFileRequest == 0.

In release mode only non-zero stats are printed by default now.

Fix diagnostic when compiler is built without statistics support.

lib/Driver/Driver.cpp

@@ -522,6 +522,7 @@ createStatsReporter(const llvm::opt::InputArgList *ArgList,
                                                  false,
                                                  false,
                                                  false,
+                                                 false,
                                                  false);
 }