Add support for llvm based intel compilers

[cpp977]

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.

[dirtyharrycallahan]

Too late for boost 1.88 but we have also hit this. Would you accept a pull request and if so do you have a preference for one of two options above?