[flang] [clang] [flang][driver] Remove Fortain_main static library from linking stages (PR #75816)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

Thanks! Mostly looks good, but I have a question:

> This leads to the problem that _QQmain and _QQEnvironmentDefaults (as of the time of this PR) are symbols marked as used, while main is being defined. 

Sorry for being pedantic, but not sure I follow. `FortranMain` defines `main`, which calls `_QQmain`. However, the latter wouldn't be available in a library, right?

 Also, not sure what defines `_QQEnvironmentDefaults ` and what exactly makes it problematic?
 
Btw, would you mind adding a note to the driver docs?

https://github.com/llvm/llvm-project/pull/75816