Fix regression from AST checkin that caused perf. warnings to be issued for stdlib code.

builtins.cpp

@@ -521,11 +521,8 @@ DefineStdlib(SymbolTable *symbolTable, llvm::LLVMContext *ctx, llvm::Module *mod
         // definitions added.  Disable emission of performance warnings for
         // now, since the user doesn't care about any of that in the stdlib
         // implementation...
-        bool epf = g->emitPerfWarnings;
-        g->emitPerfWarnings = false;
         extern char stdlib_code[];
         yy_scan_string(stdlib_code);
         yyparse();
-        g->emitPerfWarnings = epf;
     }
 }

util.cpp

@@ -295,7 +295,7 @@ Warning(SourcePos p, const char *fmt, ...) {
 
 void
 PerformanceWarning(SourcePos p, const char *fmt, ...) {
-    if (!g->emitPerfWarnings)
+    if (!g->emitPerfWarnings || strcmp(p.name, "stdlib.ispc") == 0)
         return;
 
     va_list args;