Update Classic compiler version detection for 2021. The value of the
__INTEL_COMPILER macro changed convention.
Add a special case for ifx 2021.1 to recognize it as IntelLLVM. The ifx beta versions forgot to define
__INTEL_LLVM_COMPILER, and instead define
__INTEL_COMPILER == 201900. Add a special case.
Fixes: #22120 (closed)