[lld] [ELF] Change Ctx::target to unique_ptr (PR #111260)

[via llvm-commits]

zeroomega wrote:

> > We also see a weird issue on Windows is that during clang test step, lld timed out: `Linking CXX executable unittests\Transforms\Scalar\ScalarTests.exe` took 36 minutes and caused the bot timed out. Your change is the only on the blamelist.
> 
> @zeroomega is this repeatable? We haven't seen this on our windows builds.
> 
> EDIT - we're not seeing any timeouts.. but I will check for excessive test times.

I am still digging into the timeout. 
So it probably not the linking step timeout but ninja is just waiting for some clang/lld test to finish. This is very consistent on our bot, and the timeout disappeared when the patch was reverted. I need some time to locally reproduce build to debug it further.

https://github.com/llvm/llvm-project/pull/111260