Circle CI: Use apt.llvm.org repo to install LLVM 14 on Ubuntu 20

The new min LLVM version isn't provided by Ubuntu 20 officially, so
resort to the LLVM apt repo for now, as we are stuck with Ubuntu 20
due to the gdb regressions.

Author: kinke

.circleci/config.yml

@@ -20,7 +20,13 @@ commonSteps: &commonSteps
             apt-get -yq install \
               git-core $gcc_pkg \
               zlib1g-dev $libcurl_pkg curl gdb python3 python3-pip tzdata unzip zip \
+              software-properties-common gnupg \
               $EXTRA_APT_PACKAGES
+            # set up apt.llvm.org repo for being able to install more recent LLVM versions than provided by the distro
+            curl -fsS https://apt.llvm.org/llvm-snapshot.gpg.key | tee /etc/apt/trusted.gpg.d/apt.llvm.org.asc
+            add-apt-repository -y "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-$LLVM_MAJOR main"
+            apt-get -q update
+            apt-get -yq install llvm-$LLVM_MAJOR-dev libclang-common-$LLVM_MAJOR-dev
             # Download & extract CMake
             curl -fL --retry 3 --max-time 300 -o cmake.tar.gz https://github.com/Kitware/CMake/releases/download/v3.27.1/cmake-3.27.1-linux-x86_64.tar.gz
             mkdir cmake
@@ -97,7 +103,7 @@ jobs:
     environment:
       - PARALLELISM: 4
       - CI_OS: linux
-      - EXTRA_APT_PACKAGES: llvm-11-dev libclang-common-11-dev
+      - LLVM_MAJOR: 14
       - HOST_LDC_VERSION: 1.24.0
       - EXTRA_CMAKE_FLAGS: "-DMULTILIB=ON -DRT_SUPPORT_SANITIZERS=ON -DBUILD_LTO_LIBS=ON"
   Ubuntu-20.04-sharedLibsOnly-gdmd:
@@ -108,7 +114,8 @@ jobs:
     environment:
       - PARALLELISM: 4
       - CI_OS: linux
-      - EXTRA_APT_PACKAGES: gdmd llvm-11-dev libclang-common-11-dev
+      - LLVM_MAJOR: 14
+      - EXTRA_APT_PACKAGES: gdmd
       - HOST_LDC_VERSION: 1.24.0
       - EXTRA_CMAKE_FLAGS: "-DBUILD_SHARED_LIBS=ON -DBUILD_LTO_LIBS=ON -DD_COMPILER=gdmd -DLDC_LINK_MANUALLY=ON"