Upgrade to Ubuntu 20.04

The Ubuntu 18.04 image is marked deprecated [1], so move to a newer
image.

[1] https://github.com/actions/runner-images

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@20a1fd7

Author: svenvh

llvm-spirv/.github/workflows/check-code-style.yml

@@ -29,7 +29,7 @@ env:
 jobs:
   clang-format-and-tidy:
     name: clang-format & clang-tidy
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - name: Checkout sources
       uses: actions/checkout@v2
@@ -64,7 +64,7 @@ jobs:
         # launched, so, we need to setup llvm package to perform cmake
         # configuration step to generate that database
         curl -L "https://apt.llvm.org/llvm-snapshot.gpg.key" | sudo apt-key add -
-        echo "deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic main" | sudo tee -a /etc/apt/sources.list
+        echo "deb https://apt.llvm.org/focal/ llvm-toolchain-focal main" | sudo tee -a /etc/apt/sources.list
         sudo apt-get update
         sudo apt-get install -yqq \
             clang-format-${{ env.LLVM_VERSION }} clang-tidy-${{ env.LLVM_VERSION }} \

llvm-spirv/.github/workflows/check-in-tree-build.yml

@@ -45,14 +45,14 @@ jobs:
           - build_type: Release
             shared_libs: EnableSharedLibs
       fail-fast: false
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Install dependencies
         run: |
           curl -L "https://apt.llvm.org/llvm-snapshot.gpg.key" | sudo apt-key add -
           curl -L "https://packages.lunarg.com/lunarg-signing-key-pub.asc" | sudo apt-key add -
-          echo "deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic main" | sudo tee -a /etc/apt/sources.list
-          echo "deb https://packages.lunarg.com/vulkan bionic main" | sudo tee -a /etc/apt/sources.list
+          echo "deb https://apt.llvm.org/focal/ llvm-toolchain-focal main" | sudo tee -a /etc/apt/sources.list
+          echo "deb https://packages.lunarg.com/vulkan focal main" | sudo tee -a /etc/apt/sources.list
           sudo apt-get update
           sudo apt-get -yq --no-install-suggests --no-install-recommends install \
             clang-${{ env.LLVM_VERSION }} \

llvm-spirv/.github/workflows/check-out-of-tree-build.yml

@@ -38,14 +38,14 @@ jobs:
       matrix:
         build_type: [Release, Debug]
       fail-fast: false
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Install dependencies
         run: |
           curl -L "https://apt.llvm.org/llvm-snapshot.gpg.key" | sudo apt-key add -
           curl -L "https://packages.lunarg.com/lunarg-signing-key-pub.asc" | sudo apt-key add -
-          echo "deb https://apt.llvm.org/bionic/ llvm-toolchain-bionic main" | sudo tee -a /etc/apt/sources.list
-          echo "deb https://packages.lunarg.com/vulkan bionic main" | sudo tee -a /etc/apt/sources.list
+          echo "deb https://apt.llvm.org/focal/ llvm-toolchain-focal main" | sudo tee -a /etc/apt/sources.list
+          echo "deb https://packages.lunarg.com/vulkan focal main" | sudo tee -a /etc/apt/sources.list
           sudo apt-get update
           sudo apt-get -yq --no-install-suggests --no-install-recommends install \
             clang-${{ env.LLVM_VERSION }} \