[flang] [clang] [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?

This works because I only remove `-lFortran_main` when it is going to be added implicitly. `-fno-fortran-main` ensures that we never reach this branch. I've added a test to verify this.

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