Fix failed to find project root, language server won't start on single cpp file

Hi there,
I note that if I create a single c/cpp file without `compile_commands.json` clangd server will not be started and throw this error in `:ALEInfo`
```
LSP Error Messages:
(Errors for clangd)
Failed to find project root, language server won't start.
```

However, I noticed that clangd has the option `--check=/path/to/file` for checking a single file.
Can we use `--check=` option, or add `fallbackFlags` which will be used if `compile_commands.json` does not exist?