Add --werror flag to treat warnings as errors.

The specific need for it was so that tests in tests_errors
can test to see if a desired diagnostic warning is issued
(like ptrcast-lose-info does.)

ispc.cpp

@@ -341,6 +341,7 @@ Globals::Globals() {
     runCPP = true;
     debugPrint = false;
     disableWarnings = false;
+    warningsAsErrors = false;
     disableLineWrap = false;
     emitPerfWarnings = true;
     emitInstrumentation = false;