[clang] [flang] [flang][driver] Make -stdlib= option visible to flang and silently ignored by it (PR #110598)

[David Truby via cfe-commits]

DavidTruby wrote:

I don’t personally think that the flang driver should ever attempt to link a C++ library of any kind. FWIW while there’s no stdlib option to consider in their case, g++ will never auto link the gfortran runtimes and gfortran will never auto link libstdc++

Mixed C++/Fortran programs are always going to be complicated for exactly this reason: as a user I wouldn’t expect a C++ compiler driver to automatically link a Fortran runtime or vice versa, and would find it quite surprising if that happened. In my opinion the current behaviour (which matches g++/gfortran) to require build systems to handle the complexity of adding the right linkage flags is the correct one. This would leave us rejecting stdlib= as an unrecognised option on flang as is the case in current main.

> I don't think we can rely on every build system doing that, so I would still advocate that -stdlib=libc++ is not just ignored, but actually adds the C++ standard library linker flags (see https://github.com/llvm/llvm-project/pull/110598#issuecomment-2388005207) to support such language-combination scenarios

If we were to do things this way, what should flang do if no -stdlib flag is provided? Should it link the default C++ stdlib as clang++ does, or break with clang++ and not link any C++ stdlib if nothing is provided? Both would be surprising behaviour in my opinion, and both would also risk breaking the existing cmake support.

https://github.com/llvm/llvm-project/pull/110598