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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> > 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?
> 
> I can do that.

Thank you!

> 
> I was looking around to see if one could force the use of an older libc in the test. The only approaches I found involved some unpleasant inline assembly - which probably wouldn't work in Fortran. Besides, I don't think the library organization has anything to do with glibc's symbol versioning anyway.

The comment that you added does a great job documenting the rationale for there not being a more elaborate test.

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