IntelLLVM: Fix C/C++ standard level flags on Windows

In !5594 (merged) we accidentally left out activation of the C/C++ standard level selection logic when IntelLLVM is targeting the MSVC ABI.

Fixes: #22388 (closed)
Backport: release

changed milestone to %3.21.0

added comp:intel-llvm workflow:in-review labels

Cc: @williamr

mentioned in issue #22388 (closed)

Warnings:

• Eligible for the 3.21.x branch.

The warnings do not need to be fixed, but it is recommended to do so.

Do: stage

added workflow:nightly-testing label and removed workflow:in-review label

Looks good to me. I am rebuilding to try it out.

This change worked in my test setup on Windows with icx version 2021.3.

resolved all threads

Do: merge

mentioned in commit cc654242

merged

mentioned in commit 7b942e50

removed workflow:nightly-testing label

mentioned in commit dc4b4869

mentioned in merge request !6177 (merged)

mentioned in merge request !6328 (merged)

This introduced a RunCMake.try_compile test failure in a case that needs the host CMake to agree with the tested CMake's setting of CMAKE_{C,CXX}_STANDARD_DEFAULT. This was not noticed before merging because the build that failed the test did not submit the day of the merge.

The 3.20 backport part of this MR also broke IntelLLVM C support altogether due to a bug in the way we compute the C default standard level for IntelLLVM/Clang that was fixed in post-3.20 development.

!6328 (merged) fixes both problems.

changed milestone to %3.20.6