[llvm] [mlir] [ADT] Use a dedicated empty type for StringSet (NFC) (PR #165967)

[Matt Arsenault via llvm-commits]

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@@ -2215,7 +2215,7 @@ DataAggregator::writeAggregatedFile(StringRef OutputFilename) const {
     OutFile << "boltedcollection\n";
   if (opts::BasicAggregation) {
     OutFile << "no_lbr";
-    for (const StringMapEntry<std::nullopt_t> &Entry : EventNames)
+    for (const auto &Entry : EventNames)
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arsenm wrote:

no auto? 

https://github.com/llvm/llvm-project/pull/165967