Verify that code-completion is at least a little bit robust against inclusion of missing headers

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@86551 91177308-0d34-0410-b5e6-96231b3b80d8

test/Index/code-completion.cpp

@@ -1,5 +1,5 @@
 // Code-completion through the C interface
-
+#include "nonexistent_header.h"
 struct X {
   int member;
 };