Generally, you don’t need to compile the ClangRTFM x64 project. The UE github page also says that you can compile the UE5 project (unless you know what you are doing).
If you want to solve this error, install MSBuild support for LLVM (clang-cl) toolset in the Visual Studio Installer. If you don’t have the clang compiler, install C++ Clang Compiler for Windows (18.1.8) separately.
Their load ids:
MSBuild support for LLVM (clang-cl) toolset
Microsoft.VisualStudio.Component.VC.Llvm.Clang
This should solve the error.