[llvm] [Offload] Implement the remaining initial Offload API (PR #122106)

[Callum Fare via llvm-commits]

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@@ -104,3 +104,15 @@ def : Function {
     Return<"OL_ERRC_INVALID_DEVICE">
   ];
 }
+
+def : Function {
+  let name = "olGetHostDevice";
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callumfare wrote:

This is doable but requires one of:
* Having the host device appear in each platform (the actual handle would be identical)
* Having a separate host platform with just the host device

I'm not sure which would be preferable. It's a bit clunky either way so having this function could be nice as a shortcut, although I don't know if we want to pollute the API with loads of helper functions.

https://github.com/llvm/llvm-project/pull/122106