[CI] Regression test jobs now include the C++ standard version and (if applicable) the standard library to link with

[bluetarpmedia]

• Jobs now specify the C++ standard version to compile for, and (if applicable) the standard library to link with
• Add a new job to test clang with libc++ on ubuntu. (All other ubuntu jobs for gcc & clang link with libstdc++.)
• Add a new msvc job so that we now test both c++latest and c++20.
• Reorder jobs so that GCC are first, then clang on ubuntu & macOS, and then msvc on Windows

Job names now have this format: <short os name> | <compiler> | <c++ std> | <std library> | <full os name>

The summary job list shows them like this (which unfortunately still crops the end):

Full name when you click into a job:

The test-results directories now include the C++ standard and, only for libc++ on ubuntu, the standard library name.

[hsutter]

Thanks! Sounds like this has been reviewed and is all set to go.