[clang] [llvm] [OpenMP] Add arg extensions. (PR #232938)

[Jonas Paulsson via cfe-commits]

JonPsson1 wrote:

> I'm not sure I really follow what's going on here. Can you update the PR description and explain what's going on here, what's the motivation of doing this, and what's the plan of using them?

So on some targets, like SystemZ, any argument passed (or returned) is per the C calling conventioned expected to be extended to the full register width. So passing an i32 requires an extension to i64. So when a call is emitted somewhere, it is important (for legality reasons) that such attributes are added. If missing, wrong-code problems might result as the receiving end is allowed to assume that such extensions are made (i.e. can optimize them away on their side if present). 

This is a general concern across multiple parts of the compiler, see https://github.com/llvm/llvm-project/pull/223755 where other patches are listed in completely unrelated areas but with the same concern of the arguments.

https://github.com/llvm/llvm-project/pull/232938