Fix a tablegen pattern that results in a warning; NFC

We were generating `1 || 1` which caused some issues for -Werror builds

Author: AaronBallman

clang/utils/TableGen/ClangAttrEmitter.cpp

@@ -3739,7 +3739,8 @@ static void GenerateHasAttrSpellingStringSwitch(
                       : '(' + itostr(Version) + ')';
 
     if (Scope.empty() || Scope == Spelling.nameSpace()) {
-      if (TestStringMap.contains(Spelling.name()))
+      if (TestStringMap.contains(Spelling.name()) &&
+          TestStringMap[Spelling.name()] != TestStr)
         TestStringMap[Spelling.name()] += " || " + TestStr;
       else
         TestStringMap[Spelling.name()] = TestStr;