Add llvm-hs back to stackage (#4719)

Add llvm-hs back to stackage

build-constraints.yaml

@@ -3053,7 +3053,7 @@ packages:
 
     "Moritz Kiefer <moritz.kiefer@purelyfunctional.org> @cocreature":
         - lrucaching
-        - llvm-hs < 0 # https://github.com/commercialhaskell/stackage/issues/4238
+        - llvm-hs
         - llvm-hs-pure
 
     "Thierry Bourrillon <thierry.bourrillon@fpinsight.com> @tbourrillon":

debian-bootstrap.sh

@@ -165,11 +165,11 @@ ACCEPT_EULA=Y apt-get install msodbcsql17 -y
 
 locale-gen en_US.UTF-8
 
-# llvm-7.0 for llvm-hs (separate since it needs wget)
+# llvm-8.0 for llvm-hs (separate since it needs wget)
 wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - \
     && add-apt-repository "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-7 main" \
     && apt-get update \
-    && apt-get install -y llvm-7
+    && apt-get install -y llvm-8
 
 # Buggy versions of ld.bfd fail to link some Haskell packages:
 # https://sourceware.org/bugzilla/show_bug.cgi?id=17689. Gold is