Add support for llvm based intel compilers

The new llvm based intel compilers use a different preprocessor macro `__INTEL_LLVM_COMPILER` instead of `__INTEL_COMPILER`.
This could be resolved by either adding a new compiler `BOOST_COMP_INTEL_LLVM` or by extending the current `BOOST_COMP_INTEL` part.
See https://www.intel.com/content/www/us/en/develop/documentation/oneapi-dpcpp-cpp-compiler-dev-guide-and-reference/top/compiler-reference/macros/use-predefined-macros-for-intel-compilers.html for docs about the preprocessor macros by intel.