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

[Kazu Hirata via cfe-commits]

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kazutakahirata wrote:

If you have to break up `Summary`, may I suggest structured bindings?  Note that `Summary.getSecond()` appears a few lines below:
```
for (auto &[GUID, GVS] : M.second) {
  Callback({GUID, GVS}, false);
```


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