AstGen: improve error for invalid bytes in strings and comments

test/cases/compile_errors/tab_inside_string.zig

@@ -0,0 +1,10 @@
+export fn entry() void {
+    const foo = "	hello";
+    _ = foo;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:18: error: string literal contains invalid byte: '\t'