patch 8.2.3500: Github CI fails to install clang

Problem:    Github CI fails to install clang.
Solution:   Install llvm-11 explicitly. (Christian Brabandt, closes #8993)
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 5bfcc79..9922322 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -81,7 +81,7 @@
           wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
           . /etc/lsb-release
           sudo add-apt-repository -y "deb http://apt.llvm.org/${DISTRIB_CODENAME}/ llvm-toolchain-${DISTRIB_CODENAME}-11 main"
-          sudo apt-get install -y clang-11
+          sudo apt-get install -y clang-11 llvm-11
           sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-11 100
           sudo update-alternatives --set clang /usr/bin/clang-11
           sudo update-alternatives --install /usr/bin/llvm-cov llvm-cov /usr/bin/llvm-cov-11 100