[MLIR] Apply clang-tidy fixes for misc-use-internal-linkage in AsyncRegionRewriter.cpp (NFC)

Author: joker-eph

mlir/lib/Dialect/GPU/Transforms/AsyncRegionRewriter.cpp

@@ -143,8 +143,8 @@ struct GpuAsyncRegionPass::ThreadTokenCallback {
 };
 
 /// Erases `executeOp` and returns a clone with additional `results`.
-async::ExecuteOp addExecuteResults(async::ExecuteOp executeOp,
-                                   ValueRange results) {
+static async::ExecuteOp addExecuteResults(async::ExecuteOp executeOp,
+                                          ValueRange results) {
   // Add values to async.yield op.
   Operation *yieldOp = executeOp.getBody()->getTerminator();
   yieldOp->insertOperands(yieldOp->getNumOperands(), results);