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 https://github.com/hsutter/cppfront/pull/935, which should be merged first.

[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:

• 933 fixes test failures that managed to get through some time around merging of the regression test workflow.

• 935 improves the patch generation. It has build failures (present on the main branch) that will go away after merging #933.

• This PR adds run for clang 12. It is based on #935 and should be merged after. It also has the same tests failing. Those failures will also be solved by #933.

[hsutter]

Thanks!