[clangd] Make sure ninja can clean "ClangdXPC.framework" (#75669)

After building the ClangdXPC target, `ninja clean` fails with the
following error:

```
ninja: error: remove(lib/ClangdXPC.framework): Directory not empty
ninja: error: remove(<build>/lib/ClangdXPC.framework): Directory not empty
```

I did not find better way to make this work. I guess we could list all
generated files (and directories) in `OUTPUT` of the custom command, but
that seems fairly tedious/fragile.