Fortran: Save frontend variant persistently for IntelLLVM

Since commit a90d2a9e (IntelLLVM: Add support for Intel LLVM-based
compilers, 2020-11-02, v3.20.0-rc1~89^2~20), our IntelLLVM compiler
support populates `CMAKE_Fortran_COMPILER_FRONTEND_VARIANT`.  However,
the frontend variant was not stored in `CMakeCompilerFortran.cmake`.

Signed-off-by: William R. Dieter <william.r.dieter@intel.com>