AstGen: improve error for invalid bytes in strings and comments

test/cases/compile_errors/tab_inside_multiline_string.zig

@@ -0,0 +1,13 @@
+export fn entry() void {
+    const foo =
+        \\const S = struct {
+        \\	// hello
+        \\}
+    ;
+    _ = foo;
+}
+// error
+// backend=stage2
+// target=native
+//
+// :4:11: error: string literal contains invalid byte: '\t'