[clang] [flang] [flang][Driver] Support -pthread in the frontend (PR #77360)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> Caveat: I am new to the driver and toolchain code so please let me know if I missed something! 😉

There's very few who aren't :) There's a lot of history for pretty much every driver flag - I just end up browsing documentation a lot 😂 

Here some relevant content online:
* https://developers.redhat.com/articles/2021/12/17/why-glibc-234-removed-libpthread#the_developer_view

IIUC, this means that on older system compilation will indeed fail without `-pthread`, but shouldn't be needed on newer systems. @tarunprabhu - perhaps add a link to that article in your test and add a note that on many systems compilation will succeed even without the flag?

https://github.com/llvm/llvm-project/pull/77360