In Dockerfile_base updated llvm to 12.0.0

[AndersenSemenov]

No description provided.

[korifey]

@AndersenSemenov Thanks for PR.

Couple of questions:

1. Why do we need llvm12? What problem do we solve?
2. Also, please provide test cases: how did you test your commit?

[AndersenSemenov]

@korifey

1. llvm12 provides 'continuous mode' - feature that can be used to maximize code coverage and get coverage even though program was aborted, I want to implement this in UTBot, so llvm12 is needed.
2. I wanted to open pull request and check, whether github actions would run properly, wanted to do it in my fork, but occasionally requested here.