[flang] [clang] [flang][driver] deprecate manual usage of -lFortran_main (PR #79016)

[Andrzej Warzyński via cfe-commits]

================
@@ -1193,6 +1193,16 @@ static void addFortranMain(const ToolChain &TC, const ArgList &Args,
     return;
   }
 
+  const Driver &D = TC.getDriver();
+  const char *LinkFlag = "-lFortran_main";
+
+  // warn if -lFortran_main was already specified
----------------
banach-space wrote:

```suggestion
  // Warn if the user added `-lFortran_main` - this library is an implementation detail of Flang and should be handled automatically by the driver.
```

https://github.com/llvm/llvm-project/pull/79016