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

[Tom Eccles via llvm-commits]

https://github.com/tblah commented:

Thanks for the updates. Is the speedup the same after the most recent changes?

Some of these changes look heavily dependent upon particular compiler optimizations. Which compiler are you using to build flang-rt? Is there a similar speedup for another compiler?

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