haskell-language-server hangs at 'Type checking the files'

### Your environment
Output of `haskell-language-server --probe-tools` or `haskell-language-server-wrapper --probe-tools`:

```sh
haskell-language-server version: 1.2.0.0 (GHC: 8.10.4) (PATH: /nix/store/f18pwxhycvvnsim59bzrnspgzcbq82dw-haskell-language-server-1.2.0.0/bin/haskell-language-server)
Tool versions found on the $PATH
cabal: 3.4.0.0
stack: Not found
ghc: 8.10.4
```
Which OS do you use: Debian 10

Which lsp-client do you use: N/A

Describe your project (alternative: link to the project): https://github.com/jamespwilliams/ihp-blog, this is the default project generated by `ihp-new` (IHP is https://github.com/digitallyinduced/ihp)

Contents of `hie.yaml`:
```yaml
# Used by haskell-language-server to find GHC settings
cradle:
bios:
program: build/ihp-lib/.hie-bios
```

### Steps to reproduce

Clone https://github.com/jamespwilliams/ihp-blog, run `haskell-language-server -d .`.

### Expected behaviour

Not sure, but I expect some output rather than just hanging.

### Actual behaviour

`haskell-language-server` hangs at "Type checking the files". I've left it for half-an-hour and it's made no progress.

### Include debug information
Execute in the root of your project the command `haskell-language-server --debug .` and paste the logs here:

Debug output:

```
haskell-language-server version: 1.2.0.0 (GHC: 8.10.4) (PATH: /nix/store/f18pwxhycvvnsim59bzrnspgzcbq82dw-haskell-language-server-1.2.0.0/bin/haskell-language-server)
ghcide setup tester in /home/jpw/ihp/blog.
Report bugs at https://github.com/haskell/haskell-language-server/issues

Step 1/4: Finding files to test in /home/jpw/ihp/blog
Found 15 files

Step 2/4: Looking for hie.yaml files that control setup
Found 1 cradle
(/home/jpw/ihp/blog/hie.yaml)

Step 3/4: Initializing the IDE
2021-09-21 13:44:53.214250551 [ThreadId 7] DEBUG hls: Initializing exports map from hiedb
2021-09-21 13:44:53.214589829 [ThreadId 7] DEBUG hls: Done initializing exports map from hiedb (0)

Step 4/4: Type checking the files
```

Paste the logs from the lsp-client, e.g. for [VS Code](https://github.com/haskell/vscode-haskell#troubleshooting)

LSP logs:

N/A (not actually using a LSP yet)

strace logs:

Running `strace haskell-language-server -d .` yields the following at the end:

```
write(1, "tep 4/4: Type checking the files"..., 33tep 4/4: Type checking the files
) = 33
futex(0x864b108, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0x864b110, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0x6ecd438, FUTEX_WAKE_PRIVATE, 1) = 1
futex(0x862c40c, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 0, NULL, FUTEX_BITSET_MATCH_ANY
```

So it appears to be hanging on that final futex wait.