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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> @banach-space I tentatively support this. But I am concerned that disallowing `-lFortran_main` and renaming the library will break some existing (badly configured) builds (this is not just theoretical - see my comments above). Maybe we should merge a deprecation warning on -lFortran_main before the LLVM release, then make these changes only after the release has branched?

These are very good points, +1 to all of the above!

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