Add fuzz testing of input programs.

When the --fuzz-test command-line option is given, the input program
will be randomly perturbed by the lexer in an effort to trigger
assertions or crashes in the compiler (neither of which should ever
happen, even for malformed programs.)

ispc.cpp

@@ -515,8 +515,10 @@ Globals::Globals() {
     emitPerfWarnings = true;
     emitInstrumentation = false;
     generateDebuggingSymbols = false;
+    enableFuzzTest = false;
+    fuzzTestSeed = -1;
     mangleFunctionsWithTarget = false;
-
+    
     ctx = new llvm::LLVMContext;
 
 #ifdef ISPC_IS_WINDOWS