[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179730 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/IdentifierTable.cpp b/lib/Basic/IdentifierTable.cpp
index 429d9d8..951c718 100644
--- a/lib/Basic/IdentifierTable.cpp
+++ b/lib/Basic/IdentifierTable.cpp
@@ -65,7 +65,7 @@
   };
 }
 
-IdentifierIterator *IdentifierInfoLookup::getIdentifiers() const {
+IdentifierIterator *IdentifierInfoLookup::getIdentifiers() {
   return new EmptyLookupIterator();
 }