If the identifier is unknown, return false.

[rsmith]

Patch looks good, but can you provide a better description for the commit message? (I think this is NFC but is a minor performance improvement.)

clang/lib/Serialization/GlobalModuleIndex.cpp

if (!IdentifierIndex)
     return false;
 
   // Look into the identifier index.
   ++NumIdentifierLookups;
   IdentifierIndexTable &Table
     = *static_cast<IdentifierIndexTable *>(IdentifierIndex);
   IdentifierIndexTable::iterator Known = Table.find(Name);
   if (Known == Table.end()) {
-    return true;
+    return false;
   }
 
   SmallVector<unsigned, 2> ModuleIDs = *Known;
   for (unsigned I = 0, N = ModuleIDs.size(); I != N; ++I) {
     if (ModuleFile *MF = Modules[ModuleIDs[I]].File)
       Hits.insert(MF);
   }