[Serialization] Fix a uniqueness check

[kazu]

Summary

The function is intended to maintain uniqueness of two sets -- Found
and Data. Now, we can reach the for loop shown in the patch only when
Found.empty(). This means that we never execute the body of the for
loop.

The patch fixes the problem by replacing Found with Data.

[aaron.ballman]

This looks like a sensible change to me, but it is missing test coverage. Can you add a test that demonstrates what this fixes?

clang/lib/Serialization/ASTReaderInternals.h

struct data_type_builder {
     llvm::DenseSet<DeclID> Found;
 
     data_type_builder(data_type &D) : Data(D) {}
 
     void insert(DeclID ID) {
       // Just use a linear scan unless we have more than a few IDs.
       if (Found.empty() && !Data.empty()) {
         if (Data.size() <= 4) {
-          for (auto I : Found)
+          for (auto I : Data)
             if (I == ID)
               return;
           Data.push_back(ID);
           return;
         }
 
         // Switch to tracking found IDs in the set.
         Found.insert(Data.begin(), Data.end());