haskell.compiler.ghc901: drop LLVM version to 9

GHC 9.0.1 only supports LLVM 9 and spews a lot of warnings about LLVM 10
when using the LLVM backend atm.

See also: https://www.haskell.org/ghc/download_ghc_9_0_1.html

pkgs/top-level/haskell-packages.nix

@@ -111,8 +111,8 @@ in {
           packages.ghc8107Binary;
       inherit (buildPackages.python3Packages) sphinx;
       inherit (buildPackages.darwin) autoSignDarwinBinariesHook;
-      buildLlvmPackages = buildPackages.llvmPackages_10;
-      llvmPackages = pkgs.llvmPackages_10;
+      buildLlvmPackages = buildPackages.llvmPackages_9;
+      llvmPackages = pkgs.llvmPackages_9;
     };
     ghc921 = callPackage ../development/compilers/ghc/9.2.1.nix {
       bootPkgs =