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

[Tom Eccles via cfe-commits]

tblah wrote:

> How would `flang-new -fno-fortran-main t.f -lFortran_main` work?

Good question. This won't work with this patch but currently should work. I'll have to fix this.

Do you agree that `flang-new -lFortran_main` is something we want to make work?

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