Install LLVM 9.0 in the docker image

This is required for the latest version of llvm-hs.

build-constraints.yaml

@@ -4836,10 +4836,6 @@ packages:
         # https://github.com/commercialhaskell/stackage/issues/4811
         - polysemy < 1.2.0.0
 
-        # https://github.com/commercialhaskell/stackage/issues/4812
-        - llvm-hs < 9
-        - llvm-hs-pure < 9
-
 # end of packages
 
 # Package flags are applied to individual packages, and override the values of

debian-bootstrap.sh

@@ -165,6 +165,11 @@ curl https://packages.microsoft.com/config/debian/9/prod.list > /etc/apt/sources
 apt-get update
 ACCEPT_EULA=Y apt-get install msodbcsql17 -y
 
+# llvm for llvm-hs
+curl https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
+apt-get update
+apt-get install llvm-9 -y
+
 locale-gen en_US.UTF-8
 
 # Buggy versions of ld.bfd fail to link some Haskell packages: