[Modules] Use global index to improve typo correction performance

Typo correction for an unqualified name needs to walk through all of the identifier tables of all modules.
When we have a global index, just walk its identifier table only.

rdar://13425732

llvm-svn: 179730

clang/lib/Basic/IdentifierTable.cpp

@@ -65,7 +65,7 @@ namespace {
   };
 }
 
-IdentifierIterator *IdentifierInfoLookup::getIdentifiers() const {
+IdentifierIterator *IdentifierInfoLookup::getIdentifiers() {
   return new EmptyLookupIterator();
 }