Fix old versions of CMake erroneously reporting Intel LLVM as Clang

[tobbi]

This adds a check for the specific edge case of CMAKE versions less than 3.20 reporting Intel LLVM compilers as Clang.

I've verified this using the steps described in #4315 and it works just fine.

Fixes #4315

[derekmauro]

Carrying a workaround for an unsupported platform is a maintenance burden. CMake itself doesn't even support this platform until 3.20. Here we are proposing working around an unsupported platform in a tool that doesn't support it either. Anyone serious about using CMake with IntelLLVM should just grab a copy of CMake that supports this.

[tobbi]

Maybe it would help closing the original issue as well? It really is discouraging when you fix an issue that you ("Google") supposedly need help with only to be met with this message.

[derekmauro]

Maybe it would help closing the original issue as well? It really is discouraging when you fix an issue that you ("Google") supposedly need help with only to be met with this message.

Sorry about that. I apologize for asking for help, but please realize that it wasn't until I saw your patch that I realized that it would have been best to just suggest upgrading CMake.