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

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

Thanks, I am not sure how I missed that. 

@banach-space, Is the intention for the `-pthread` option to be illegal in flang?

At the last flang community call ([2024-01-03](https://docs.google.com/document/d/1Z2U5UAtJ-Dag5wlMaLaW1KRmNgENNAYynJqLW2j2AZQ/edit#heading=h.de8wft8m7vbj)), one user reported being unable to build with OpenMPI because it inserted the -pthread flag. I suspect that some application build systems do the same thing, but I can't point to an example off the top of my head. 

CC'ing @ergawy as the original author.

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