The changes are part of CMP0128.
When the standard level is unset:
* Flags are added if extension mode doesn't match the compiler's default.
Previously logic only worked if LANG_EXTENSIONS was ON. Fixes#22224.
* The full flag is used. Previously CMAKE_LANG_EXTENSION_COMPILE_OPTION was
used. This was only supported for IAR.
Otherwise:
* Avoid adding flags if not necessary per the detected compiler defaults.
* Fixed check for when the requested standard is older. It now matches the
nearby comments.
I reworded the fallback comment as its logic was a bit difficult to wrap my
head around.
Remove the Cuda/ConsumeCompileFeatures test as it only existed to
validate that using `cxx_std_XY` would cause the paired language (CUDA) to also build in mode XY.
The cmLocalGenerator::AddCompilerRequirementFlag was another
source of truth on what standard levels existed per language,
so fold that into cmStandardLevelResolver to make updating
CMake easier
