Fortran: Save frontend variant persistently for IntelLLVM

Since commit a90d2a9eed (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 <[email protected]>

Modules/CMakeFortranCompiler.cmake.in

@@ -5,6 +5,7 @@ set(CMAKE_Fortran_COMPILER_VERSION "@CMAKE_Fortran_COMPILER_VERSION@")
 set(CMAKE_Fortran_COMPILER_WRAPPER "@CMAKE_Fortran_COMPILER_WRAPPER@")
 set(CMAKE_Fortran_PLATFORM_ID "@CMAKE_Fortran_PLATFORM_ID@")
 set(CMAKE_Fortran_SIMULATE_ID "@CMAKE_Fortran_SIMULATE_ID@")
+set(CMAKE_Fortran_COMPILER_FRONTEND_VARIANT "@CMAKE_Fortran_COMPILER_FRONTEND_VARIANT@")
 set(CMAKE_Fortran_SIMULATE_VERSION "@CMAKE_Fortran_SIMULATE_VERSION@")
 @_SET_CMAKE_Fortran_XL_CPP@
 @_SET_CMAKE_Fortran_COMPILER_ARCHITECTURE_ID@