[clang] [Clang][Driver] Revise Cygwin ToolChain to call linker directly (PR #147960)

[via cfe-commits]

tyan0 wrote:

> This will need more tests added to cover these additional flags that are now hooked up. Off the top of my head, `-shared`/`-mdll` and the `_cygwin_dll_entry` x86_64 vs i686, `-mconsole`/`-mwindows`, the high-entropy-va thing. Might also be worthwhile to look at the hurd.cpp driver test and see if any more of that is relevant now, that's what I started with to make the cygwin.cpp test.
> 
> Is the `_cygwin_dll_entry` option being added for `-mdll` or only for `-shared`? It looks like gcc does it for both, but only changes to `crtbeginS.o` for `-shared`

Thanks. Test updated.

https://github.com/llvm/llvm-project/pull/147960