CMakeParseImplicitLinkInfo: Exclude LLVMFlang program entry point for MSVC

As of llvm-project `main` branch commit `86accd4e03` (2023-12-04),
LLVMFlang 18.0.0, when used to drive linking an executable, emits a MSVC
linker flag to use all object files from the `Fortran_main` library.
These object files are meant for use when linking the program entry
point, and so are not implicit link dependencies of Fortran libraries.