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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

The problem is that clang is more forgiving in that it will generate the default template instantiation even if you don't explicitly ask for it, whereas gcc does not and it then errors out when you try to use it. This fixes it for me:
https://github.com/llvm/llvm-project/pull/141619

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