[clang] Fix a dangling reference in clang/utils/TableGen/ClangDiagnosticsEmitter.cpp (llvm#119197)

`DiagsInGroup` is a `map<llvm::StringRef, ...>`, we store a dangling
string_view in the key.

Author: hokein

clang/utils/TableGen/ClangDiagnosticsEmitter.cpp

@@ -1907,8 +1907,7 @@ void clang::EmitClangDiagDocs(const RecordKeeper &Records, raw_ostream &OS) {
   // Write out the diagnostic groups.
   for (const Record *G : DiagGroups) {
     bool IsRemarkGroup = isRemarkGroup(G, DiagsInGroup);
-    auto &GroupInfo =
-        DiagsInGroup[std::string(G->getValueAsString("GroupName"))];
+    auto &GroupInfo = DiagsInGroup[G->getValueAsString("GroupName")];
     bool IsSynonym = GroupInfo.DiagsInGroup.empty() &&
                      GroupInfo.SubGroups.size() == 1;