[clang] [flang] [flang][driver] Do not allow -module-dir to be joined to the value (PR #168748)

[Michael Kruse via cfe-commits]

Meinersbur wrote:

As much as I would like this, I fear CMake has to be fixed first: https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/Compiler/LLVMFlang-Fortran.cmake?ref_type=heads#L10

https://github.com/llvm/llvm-project/pull/168748