[llvm] [flang-rt] Optimise ShallowCopy and use it in CopyInAssign (PR #140569)

[Slava Zakharin via llvm-commits]

https://github.com/vzakhari commented:

Thanks for the update!

Please add unittests in `flang-rt/unittests/Runtime`.

https://github.com/llvm/llvm-project/pull/140569