From Vassil Vassilev: enable Sema to deal with multiple ExternalSemaSources.

llvm-svn: 166208
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 20fa279..489e0a6 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -5564,7 +5564,7 @@
 
 void ASTReader::InitializeSema(Sema &S) {
   SemaObj = &S;
-  S.ExternalSource = this;
+  S.addExternalSource(this);
 
   // Makes sure any declarations that were deserialized "too early"
   // still get added to the identifier's declaration chains.