CI: Add clang++-12 regression tests run on Ubuntu

[jarzec]

The current regression tests workflow is missing a run using clang++-12.
This PR adds this compiler and updates the respective tests.

Note that this branch is based on #935, which should be merged first.

[jarzec]

clang++-12 is not installed by default on the ubuntu-latest image, but it is on ubuntu-20.04, so this is the easiest way to include it.

[hsutter]

Thanks! @JohelEGP @jarzec @bluetarpmedia I see a few apparently-related PRs by the three of you, is there an order they should be merged?

[JohelEGP]

By their PR number should work OK.

[JarekGlobus]

Thanks! @JohelEGP @jarzec @bluetarpmedia I see a few apparently-related PRs by the three of you, is there an order they should be merged?

@hsutter To be more precise:

• Fix macOS and Ubuntu regression tests #933 fixes test failures that managed to get through some time around merging of the regression test workflow.
• ci: apply review comments on patch upload #935 improves the patch generation. It has build failures (present on the main branch) that will go away after merging Fix macOS and Ubuntu regression tests #933.
• This PR adds run for clang 12. It is based on ci: apply review comments on patch upload #935 and should be merged after. It also has the same tests failing. Those failures will also be solved by Fix macOS and Ubuntu regression tests #933.

[hsutter]

Thanks!