[Ubuntu] Default Clang will be set to Clang 11 on December, 14

[AlenaSviridenko]

Breaking changes
Default Clang will be set to Clang 11 on Ubuntu 20.04 image (PR #2087).

Target date
Image deployment will start on December, 7 December, 14 and will take 2-3 days

The motivation for the changes
We'd like to provide the image with the latest actual software provided by default.

Possible impact
If your project is not compatible with Clang 11 and you are using default Clang - your build will be broken.

Virtual environments affected

• Ubuntu 16.04
• Ubuntu 18.04
• Ubuntu 20.04
• macOS 10.13
• macOS 10.14
• macOS 10.15
• macOS 11.0
• Windows Server 2016 R2
• Windows Server 2019

Mitigation ways
Run the following script to set Clang 10 as a default version explicitly:

sudo update-alternatives --remove-all clang
sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-10 100
sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-10 100
sudo update-alternatives --install /usr/bin/clang-format clang-format /usr/bin/clang-format-10 100

[maxim-lobanov]

We have to postpone this update for now since Clang 11 is not available for Ubuntu 20.04 in official apt repo yet.
Will revisit it later.

[dmitryikh]

@maxim-lobanov , currently I install clang11 for ubuntu-latest

sudo apt-get install clang-11 libc++-11-dev libc++abi-11-dev

seems like clang11 is landed to official apt repo.

[miketimofeev]

@dmitryikh thanks for the update! Could you please create an issue if you need clang11 on the image?