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

[Tom Eccles via cfe-commits]

tblah wrote:

> My suggestion:
> 
>     * rename `Fortran_main.a` as e.g. `flang_fortran_main.a`,
> 
>     * introduce `-ffortran-main` on top of `-fno-fortran-main`,
> 
>     * disallow `-lFortran_main.a` and `-lflang_fortran_main.a` - this library should be kept as an implementation detail of Flang that can change in the future.

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?

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