[clang] [NFC][Clang] Use StringRef instead of string in ClangDiagnosticEmitter (PR #115959)

[Kazu Hirata via cfe-commits]

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@@ -145,7 +146,7 @@ static bool diagGroupBeforeByName(const Record *LHS, const Record *RHS) {
          RHS->getValueAsString("GroupName");
 }
 
-using DiagsInGroupTy = std::map<std::string, GroupInfo, std::less<>>;
+using DiagsInGroupTy = std::map<StringRef, GroupInfo, std::less<>>;
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kazutakahirata wrote:

If you are putting `StringRef`, I'd drop `std::less<>`.  We don't need to support heterogenous lookups if the key is of `StringRef`.

https://github.com/llvm/llvm-project/pull/115959