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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

I am not too fond of this.

The current design has taken quite some effort and a few discussions to land in its current form. Also, Flang has gained proper documentation for this:

* https://github.com/llvm/llvm-project/blob/main/flang/docs/FlangDriver.md#linker-driver

This change is effectively undoing an important part of that. In this case:
```
flang-new -lFortran_main 
```
the right thing to do would be to check the documentation, which should explain: "don't do this, because of xyz".

In general, I see `Fortran_Main` as an implementation detail that end users should not require and I like the idea of "hiding" it from the end user.

Having said all that, I'm not going to block this. Howerver, I would like to see that there's consensus before this lands.

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