[llvm] [offload] Add `dlwrap::loaded` function to check for optional symbols (PR #210737)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-offload

Author: blazej-smorawski

<details>
<summary>Changes</summary>

This PR adds a new template into `dlwrap` namespace that can be used to check if a symbol was correctly loaded. It adds and easy way to see if version of shared object in a system has required capability. We could use it to improve prefetch in CUDA backend as noted [here](https://github.com/llvm/llvm-project/blob/main/offload/plugins-nextgen/cuda/src/rtl.cpp#L912) without breaking compatibility with older platforms using CUDA older than 13.

In the case of prefetch, the new `dlwrap` API could be used like: 
```cpp
bool BatchedPrefetchAvailable = dlwrap::loaded<cuMemPrefetchBatchAsync>();
if (BatchedPrefetchAvailable)
    cuMemPrefetchAsync(....)
else
    // Current implementation
```

Small note, to implement the prefetch we will also have to make loading of the symbols optional inside plugins too. This would require to change it [here](https://github.com/llvm/llvm-project/blob/main/offload/plugins-nextgen/cuda/dynamic_cuda/cuda.cpp#L176), but I think that the change can be a part of a PR that implements prefetch.

---
Full diff: https://github.com/llvm/llvm-project/pull/210737.diff


1 Files Affected:

- (modified) offload/plugins-nextgen/common/include/DLWrap.h (+5) 


``````````diff
diff --git a/offload/plugins-nextgen/common/include/DLWrap.h b/offload/plugins-nextgen/common/include/DLWrap.h
index 95ce86e123cd3..116cd4afe37a6 100644
--- a/offload/plugins-nextgen/common/include/DLWrap.h
+++ b/offload/plugins-nextgen/common/include/DLWrap.h
@@ -135,6 +135,8 @@ template <size_t Requested, size_t Required> constexpr void verboseAssert() {
   static_assert(Requested == Required, "Arity Error");
 }
 
+template <auto Fn> bool loaded();
+
 } // namespace dlwrap
 
 #define DLWRAP_INSTANTIATE(SYM_DEF, SYM_USE, ARITY)                            \
@@ -167,6 +169,9 @@ template <size_t Requested, size_t Required> constexpr void verboseAssert() {
       return reinterpret_cast<T::FunctionType>(P);                             \
     }                                                                          \
   };                                                                           \
+  template <> bool loaded<&(::SYMBOL)>() {                                     \
+    return SYMBOL##_Trait::get() != nullptr;                                   \
+  }                                                                            \
   }
 
 #define DLWRAP_IMPL(SYMBOL, ARITY)                                             \

``````````

</details>


https://github.com/llvm/llvm-project/pull/210737