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

[Ilya Biryukov via cfe-commits]

================
@@ -1908,7 +1908,7 @@ void clang::EmitClangDiagDocs(const RecordKeeper &Records, raw_ostream &OS) {
   for (const Record *G : DiagGroups) {
     bool IsRemarkGroup = isRemarkGroup(G, DiagsInGroup);
     auto &GroupInfo =
-        DiagsInGroup[std::string(G->getValueAsString("GroupName"))];
----------------
ilya-biryukov wrote:

Maybe replace with `llvm::StringMap` to get better performance and avoid these problems for good?
I would be surprised if the `StringRef` in the map is critical for performance and I am 99% certain that `StringMap` will likely win over `std::map<StringRef, ...` anyway.

https://github.com/llvm/llvm-project/pull/119197