[clang] [clangl[TableGen] Change Diagnostic Emitter to use const RecordKeeper (PR #108209)

[Rahul Joshi via cfe-commits]

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@@ -68,7 +69,8 @@ getCategoryFromDiagGroup(const Record *Group,
 
   // The diag group may the subgroup of one or more other diagnostic groups,
   // check these for a category as well.
-  const std::vector<Record*> &Parents = DiagGroupParents.getParents(Group);
+  const std::vector<const Record *> &Parents =
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jurahul wrote:

We could, I was trying to not change more than what's necessary for this change. I think there are other cleanup like this that can done for all the clang tablegen backends (like using ArrayRef instead of const ref to vectors, range based for loops etc). I was thinking that those can be done in a separate NFC (I can do that once this series of patches goes in). 

https://github.com/llvm/llvm-project/pull/108209