cabal haddock fails for foreign libraries

When building the docs for the package in `cabal-testsuite/PackageTests/ForeignLibs`:

```
$ cabal haddock
Resolving dependencies...
Configuring my-foreign-lib-0.1.0.0...
Running Haddock for my-foreign-lib-0.1.0.0...
Preprocessing library my-foreign-lib-0.1.0.0...
Haddock coverage:
0% ( 0 / 2) in 'MyForeignLib.AnotherVal'
Missing documentation for:
Module header
anotherVal (src/MyForeignLib/AnotherVal.hs:3)
Documentation created: dist/doc/html/my-foreign-lib/index.html
Preprocessing foreign library 'myforeignlib' for my-foreign-lib-0.1.0.0...
ppHsc2hs: broken closure: []
CallStack (from HasCallStack):
error, called at ./Distribution/Simple/PreProcess.hs:457:17 in
Cabal-1.25.0.0-LJMdZv6ScFWHI53bE5EyDD:Distribution.Simple.PreProcess
```