[clang] [flang] [flang][Driver] Accept (and ignore) some gfortran-specific options (PR #165579)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> I think this needs a quick RFC just to make everyone aware and see that there are no strong objections.

There do not seem to be any objections to the RFC. Can we proceed with this PR? 

A related [RFC](https://discourse.llvm.org/t/rfc-command-line-compatibility-with-gfortran/88961/1)  has been posted regarding blanket command-line compatibility with `gfortran`. This PR, on the other hand, is about compatibility with `clang`, which is, arguably, more desirable.

https://github.com/llvm/llvm-project/pull/165579