[clang] [Driver] Handle Flang in same manner between Gnu and *BSD/Solaris ToolChain (PR #70429)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

@brad0 Thanks for sending this - could you provide some context in the summary? Why update the GNU Toolchain rather than *BSD  - as in, what should be source of truth here? Also, could you add tests?

I will be OK next week, but am happy for this to be merged once my comments are addressed and @MaskRay is also happy with this. Thanks!

https://github.com/llvm/llvm-project/pull/70429