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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> I wonder why they decided to diverge from gfortan-style flags:
> https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/Compiler/GNU-Fortran.cmake#L24

https://discourse.llvm.org/t/rfc-new-flang-driver-options/56960/3?

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