LLVMFlang: Add support for LLVM Flang

LLVM Flang (https://github.com/llvm/llvm-project/tree/main/flang) is an LLVM
Fortran compiler that shares the same name as Flang (also known as Classic
Flang). Classic Flang is in active development and is already identified by
CMake as Flang. As such, LLVM Flang will be identified as `LLVMFlang`.

Fixes: #22387
5 files changed