Exclude tab in string literals (#734)

Noticed this while working on #732, seemed easiest to split changes though.

executable_semantics/testdata/string_fail6.carbon

@@ -0,0 +1,8 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn main() -> i32 {
+  Print("new	line");
+  return 0;
+}