CI: Add Clang 15 support in regression tests

[jarzec]

Add support for Clang 15 in regression tests.

• Include the Homebrew version of Clang 15 as supported compiler.
  • The test results are identical as for Apple Clang 14 - the existing test-results/apple-clang-14 directory can be reused.
• Switch the default C++ standard used for regression test compilation to c++2b. Stick to c++20 for:
  • GCC 10 that simply does not support c++2b. Due to this lack of support this version might possibly be dropped leaving only GCC 13.
  • Clang 15 on Ubuntu gives strange issues with c++2b - to be investigated further.

NOTE:

• The switch to c++2b makes the lowered pure2-assert-expected-not-null.cpp2 compile for GCC 13.
• The result dir test-results/clang-15 added by this PR differs from test-results/apple-clang-14 only for:
  • mixed-captures-in-expressions-and-postconditions.cpp2
  • pure2-type-and-namespace-aliases.cpp2
• While preparing the PR I was able to confirm the spurious selection of Apple clang version 15.0.0 in some runs. Unfortunately the solution using xcodebuild -find clang++ doesn't seem to help as can be seen in this build that tested another approach to use that command. It seem that is such cases XCode 15 is used by default. The problem will require further investigation or a fix of the GitHub macos-13 runner. Re-running jobs failing due to this issue will remain a workaround. I've added a command to select Xcode 14. This should hopefully solve the issue.

REMARK: Ubuntu 2404 will likely come out with Clang up to v18 and GCC up to v14 available directly from apt, without the need to install them manually (see here). This might be a good point to start looking into including newer compilers with better C++23 support and possibly dropping those without any (namely GCC 10).

[hsutter]

Thanks! I was about to hit Merge but I see there are more commits -- LMK when this is ready.

[hsutter]

Great, thanks again.