[llvm] [OpenMP][OpenMPIRBuilder][NFC] Move copyInput to a passed in lambda function (PR #68124)

[via llvm-commits]

agozillon wrote:

Updated to utilise the InsertPoint style interface for the lambda, hopefully it's a little more inline with what is expected for the CallbackInterfaces! Please let me know if it's not and if there's any other changes you'd like made.

I also tried to generate output a little more like Clang, with the Allocas/Store preamble for the argument in the entry block and the loads in the user block. You can see what I mean a bit more tangibly with the altered tests. If we're not comfortable with that I can simply use the same InsertPoint for loads/allocas/stores as before. However, this change doesn't affect any runtime tests we have at the moment. 

https://github.com/llvm/llvm-project/pull/68124