[llvm] [NFC] Use more efficient data structures in SubtargetEmitter (PR #102090)

[Nikita Popov via llvm-commits]

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@@ -260,7 +260,7 @@ unsigned SubtargetEmitter::FeatureKeyValues(
   llvm::sort(FeatureList, LessRecordFieldName());
 
   // Check that there are no duplicate keys
-  std::set<StringRef> UniqueKeys;
+  llvm::StringSet<> UniqueKeys;
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nikic wrote:

This should use `DenseSet<StringRef>`, not `StringSet`. Only use StringSet if the set must own the strings.

https://github.com/llvm/llvm-project/pull/102090