[clang] [HLSL] Run availability diagnostic on exported functions (PR #97352)

[Xiang Li via cfe-commits]

================
@@ -129,6 +129,55 @@ class MyClass
   }
 };
 
+// Exported function without body - not used
+export void exportedFunctionUnused(float f);
----------------
python3kgae wrote:

Will there be an error if has export when declare like
```
export void export_mismatch(float f);
```
But without export when define like
```
void export_mismatch(float f) { return f; }
```

https://github.com/llvm/llvm-project/pull/97352