[TableGen] Use ListSeparator (NFC)

Note that an instance of ListSeparator evaluates to the empty string
for the first time and then ", " for subsequent references.

Author: kazutakahirata

clang/utils/TableGen/ClangAttrEmitter.cpp

@@ -5482,14 +5482,12 @@ void EmitTestPragmaAttributeSupportedAttributes(const RecordKeeper &Records,
     }
     const Record *SubjectObj = I.second->getValueAsDef("Subjects");
     OS << " (";
-    bool PrintComma = false;
+    ListSeparator LS;
     for (const auto &Subject :
          enumerate(SubjectObj->getValueAsListOfDefs("Subjects"))) {
       if (!isSupportedPragmaClangAttributeSubject(*Subject.value()))
         continue;
-      if (PrintComma)
-        OS << ", ";
-      PrintComma = true;
+      OS << LS;
       PragmaClangAttributeSupport::RuleOrAggregateRuleSet &RuleSet =
           Support.SubjectsToRules.find(Subject.value())->getSecond();
       if (RuleSet.isRule()) {

clang/utils/TableGen/ClangDiagnosticsEmitter.cpp

@@ -2225,13 +2225,10 @@ void clang::EmitClangDiagDocs(const RecordKeeper &Records, raw_ostream &OS) {
       else
         OS << "Also controls ";
 
-      bool First = true;
       sort(GroupInfo.SubGroups);
-      for (StringRef Name : GroupInfo.SubGroups) {
-        if (!First) OS << ", ";
-        OS << "`" << (IsRemarkGroup ? "-R" : "-W") << Name << "`_";
-        First = false;
-      }
+      ListSeparator LS;
+      for (StringRef Name : GroupInfo.SubGroups)
+        OS << LS << "`" << (IsRemarkGroup ? "-R" : "-W") << Name << "`_";
       OS << ".\n\n";
     }