switches to a non-alternate llvm, removes the LLVM_CONFIG step

First of all, `/usr/local/opt/llvm/bin/llvm-config`
is llvm-config of LLVM that is shipped with MacOS and which is
stripped from any non-native targets so we can't use it. The
requested llvm@9 is installed at `/usr/local/opt/llvm@9` and is not
symlinked to `/usr/local` because it is keg-only:

```
llvm@9 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.

If you need to have llvm@9 first in your PATH run:
  echo 'export PATH="/usr/local/opt/llvm@9/bin:$PATH"' >> /Users/runner/.bash_profile

For compilers to find llvm@9 you may need to set:
  export LDFLAGS="-L/usr/local/opt/llvm@9/lib"
  export CPPFLAGS="-I/usr/local/opt/llvm@9/include"
```

llvm@10 aka llvm is also keg-only so it will be installed only into
the cellar. I have to guess whether the llvm-config binary will be
named llvm-config or llvm-config-9, but I bet for the former.

Author: ivg

.github/workflows/main.yml

@@ -36,7 +36,7 @@ jobs:
 
       - name: Configure Homebrew LLVM
         if: matrix.os == 'macos-latest'
-        run: echo '::set-env name=LLVM_CONFIG::/usr/local/opt/llvm/bin/llvm-config'
+        run: echo '::set-env name=LLVM_CONFIG::/usr/local/opt/llvm@9/bin/llvm-config'
 
       - name: Install system dependencies
         run: opam depext bap --yes