[flang] [clang] [flang][driver] Allow explicit specification of -lFortran_main (PR #78152)

[Michael Klemm via cfe-commits]

mjklemm wrote:

> Maybe we should merge a deprecation warning on -lFortran_main before the LLVM release, then make these changes only after the release has branched?

That sounds like a good idea for now.  Then, we can buy a bit of time for the RFC and to make decision how we want to deal with the `main` problem (pun intended :-)).

https://github.com/llvm/llvm-project/pull/78152