[clang-tblgen] Fix non-determinism in generating AttributeReference.rst

[ikudrin]

Summary

As for now, the categories are printed in an arbitrary order which depends on the addresses of dynamically allocated objects. The patch sorts them in an alphabetical order thus making the output stable.

[aaron.ballman]

LGTM, thank you for the fix!

clang/utils/TableGen/ClangAttrEmitter.cpp

if (!Documentation) {
     return;
   }
 
   OS << Documentation->getValueAsString("Intro") << "\n";
 
   // Gather the Documentation lists from each of the attributes, based on the
   // category provided.
   std::vector<Record *> Attrs = Records.getAllDerivedDefinitions("Attr");
-  std::map<const Record *, std::vector<DocumentationData>> SplitDocs;
+  struct CategoryLess {
+    bool operator()(const Record *L, const Record *R) const {
+      return L->getValueAsString("Name") < R->getValueAsString("Name");
+    }
+  };
+  std::map<const Record *, std::vector<DocumentationData>, CategoryLess>
+      SplitDocs;
   for (const auto *A : Attrs) {
     const Record &Attr = *A;
     std::vector<Record *> Docs = Attr.getValueAsListOfDefs("Documentation");
     for (const auto *D : Docs) {
       const Record &Doc = *D;
       const Record *Category = Doc.getValueAsDef("Category");
       // If the category is "undocumented", then there cannot be any other
       // documentation categories (otherwise, the attribute would become