Remove linux container and use clang 9

pwsh is not detected inside the container, and Clang 5 is very old.

script/vsts/platforms/linux.yml

@@ -7,7 +7,6 @@ jobs:
     pool:
       # This image is used to host the Docker container that runs the build
       vmImage: ubuntu-16.04
-    container: ubuntu:trusty
 
     steps:
       - template: templates/preparation.yml

script/vsts/platforms/templates/bootstrap.yml

@@ -20,7 +20,7 @@ steps:
       ${{ if in(variables['Agent.OS'], 'Darwin', 'Windows_NT') }}:
         npm_config_build_from_source: true
       ${{ if eq(variables['Agent.OS'], 'Linux') }}:
-        CC: clang-5.0
-        CXX: clang++-5.0
+        CC: clang
+        CXX: clang++
         npm_config_clang: 1
       condition: or(ne(variables['MainNodeModulesRestored'], 'true'), ne(variables['ScriptNodeModulesRestored'], 'true'), ne(variables['ApmNodeModulesRestored'], 'true'))

script/vsts/platforms/templates/linux-build.yml

@@ -3,8 +3,8 @@ steps:
     env:
       GITHUB_TOKEN: $(GITHUB_TOKEN)
       ATOM_RELEASE_VERSION: $(ReleaseVersion)
-      CC: clang-5.0
-      CXX: clang++-5.0
+      CC: clang
+      CXX: clang++
       npm_config_clang: 1
     displayName: Build Atom
 

script/vsts/platforms/templates/preparation.yml

@@ -4,13 +4,17 @@ steps:
   - script: |
       sudo apt-get update
       sudo apt-get install -y wget software-properties-common
-      wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-      sudo apt-add-repository "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-5.0 main"
-      sudo apt-get update
-      sudo apt-get install -y build-essential ca-certificates clang-5.0 xvfb fakeroot git libsecret-1-dev rpm libx11-dev libxkbfile-dev xz-utils xorriso zsync libxss1 libgconf2-4 libgtk-3-0 libasound2 libicu-dev
+      sudo apt-get install -y build-essential ca-certificates xvfb fakeroot git libsecret-1-dev rpm libx11-dev libxkbfile-dev xz-utils xorriso zsync libxss1 libgconf2-4 libgtk-3-0 libasound2 libicu-dev
+      # clang 9 is included in the image
+      sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-9 10
+      sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-9 10
+      sudo update-alternatives --config clang
+      sudo update-alternatives --config clang++
+      clang -v
     displayName: Install apt dependencies
     condition: eq(variables['Agent.OS'], 'Linux')
 
+
   - script: sudo /sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1280x1024x16
     displayName: Start Xvfb
     condition: eq(variables['Agent.OS'], 'Linux')