Make ChainedIncludesSource an ExternalSemaSource, otherwise initialization of the ASTReader is incomplete, leading to errors like not realizing std::type_info is already defined.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@128664 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaExprCXX.cpp b/lib/Sema/SemaExprCXX.cpp
index 36079c2..8c81acc 100644
--- a/lib/Sema/SemaExprCXX.cpp
+++ b/lib/Sema/SemaExprCXX.cpp
@@ -343,7 +343,7 @@
 Sema::ActOnCXXTypeid(SourceLocation OpLoc, SourceLocation LParenLoc,
                      bool isType, void *TyOrExpr, SourceLocation RParenLoc) {
   // Find the std::type_info type.
-  if (!StdNamespace)
+  if (!getStdNamespace())
     return ExprError(Diag(OpLoc, diag::err_need_header_before_typeid));
 
   if (!CXXTypeInfoDecl) {