[clang] [llvm] [mlir] [ADT] Give DenseMapPair its own members instead of a std::pair base. NFC (PR #221853)

[Kazu Hirata via llvm-commits]

================
@@ -760,9 +760,8 @@ ParallelToGpuLaunchLowering::matchAndRewrite(ParallelOp parallelOp,
 
   // Now that we succeeded creating the launch operation, also update the
   // bounds.
-  for (auto bound : launchBounds)
-    launchOp.setOperand(getLaunchOpArgumentNum(std::get<0>(bound)),
-                        std::get<1>(bound));
+  for (const auto &bound : launchBounds)
+    launchOp.setOperand(getLaunchOpArgumentNum(bound.first), bound.second);
----------------
kazutakahirata wrote:

How about structured bindings here?
```suggestion
for (const auto &[processor, bound] : launchBounds)
  launchOp.setOperand(getLaunchOpArgumentNum(processor), bound);
```

https://github.com/llvm/llvm-project/pull/221853