Since !6319 (merged) we activate C/C++ standard level logic for IntelLLVM when targeting the MSVC ABI. Update the RunCMake.try_compile test to be aware of this even when CMake is itself configured by an older CMake that does not know this.
Also fix the 3.20 backport of !6319 (merged) by also backporting commit 7596d8b9 from !5783 (merged).
Backport: release