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

[Tarun Prabhu via cfe-commits]

tarunprabhu 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. 

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.


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