Clang dependency for tests

[hgsipiere]

I think we should mention the clang dependency for tests on the test-data, idris-grin.

I have a passing stack build and failing stack build on Fedora 31 with LLVM 7 installed but not Clang. I would have put in a pull request but I don't have a Mac or Debian-based computer setup.

[andorp]

I can validate and/or adapt the PR to debian if you raise one.

[hgsipiere]

Okay thank you, I'll actually just setup a Debian container and submit a pull request when I have some free time.