[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:

Or maybe it needs two callbacks actually, but I think it would be nicer to do with a single API call anyway

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