Add C++ standards to CI config (Linux)

[alexezeder]

For Linux workflow, I added several C++ standards with excluding unsupported/excess standard for each presented compiler.
This is related to the issue #1987.
Also, I fixed 1 compile warning (-Werror,-Wsign-conversion) for clang++ with C++17 standard, otherwise, CI build fails.

[darklukee]

Doesn't mean someone won't use c++11 on gcc-8.
If resources aren't a constrain I wouldn't exclude it from checks.

Same comment for other excludes like this one.

[alexezeder]

I was just impressed by the number of configurations without any filtering.
Maybe I over-filtered them.
But I don't know if they make any sense, for example, is it possible that c++11 on gcc-8 fails while c++11 on gcc-4.8 doesn't? It can be possible of course, because of some bugs, but if we are hunting for bugs here then this config should have a huge number of configurations.

[darklukee]

I see your point.
However following this though, standard shouldn't be a matrix parameter.
Check default on each compiler and have separate matrix for all cpp standards tested on newest compilers only.

[alexezeder]

Ok, this is a good suggestion.

[alexezeder]

But I think it will be better to create a new PR with the described logic because the logic here and the logic you proposed differ. I can create it, or maybe you want to do this?

[jgopel]

A quick note here in case this is preferred over #1999: You may want to consider disabling C++20 support on gcc-8 builds. gcc-8 only has partial support for 20 and it will never fully support it. The GCC folks have kindly documented this for us here: https://gcc.gnu.org/projects/cxx-status.html.

[alexezeder]

@jgopel hmm... but C++20 on gcc-8 is disabled.

I'm closing this PR. #1999 is better because it has additional fixes.

I think the most important thing in this PR is the idea proposed by @darklukee that standard shouldn't be a matrix parameter

Following this idea I would make a configuration with several jobs inside, like:

jobs:
  cxx11:
    ...

  cxx14:
    ...

  cxx17_cxx20: # it's combined because no compilers have c++17 as default standard, better be separated
    ...

Where the cxx11 job should run on ubuntu 16.04 to use the lowest possible compilers: gcc-4.8 (also available on 18.04) and clang-3.5. Maybe instead of this prepared image, this job should use a Docker container with gcc-4.8 and clang-3.0. Why these compilers? Because the documentation says:

These are available in GCC 4.8, Clang 3.0, MSVC 19.0 (2015) and more recent compiler version. For older compilers use {fmt} version 4.x which is maintained and only requires C++98.

Off-top: what a shame that GitHub Action doesn't support YAML anchors.

[jgopel]

@jgopel hmm... but C++20 on gcc-8 is disabled.

I totally missed the additional blocks after the gcc-4.8 block - I stand corrected