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

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

Thanks for the updates!
LGTM now.

> Now that the other PR has been merged, we can add some tests for `-rtlib=compiler-rt`.

Just to clarify, did you decide not to add any tests? (either is fine with me.)

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