[python] [tests] Fix test_linkage for unique external linkage

Starting with r314037, anonymous namespaces no longer give
unique-external linkage to variables. However, this linkage can still be
achieved by using a type which is not exterally visible,
e.g. through being declared in an anonymous namespace but used outside
it. Fix the test to take advantage of that.

Differential Revision: https://reviews.llvm.org/D39810

llvm-svn: 317986
diff --git a/clang/bindings/python/tests/cindex/test_linkage.py b/clang/bindings/python/tests/cindex/test_linkage.py
index 37223c2..6b482f8 100644
--- a/clang/bindings/python/tests/cindex/test_linkage.py
+++ b/clang/bindings/python/tests/cindex/test_linkage.py
@@ -15,7 +15,8 @@
         tu = get_tu("""
 void foo() { int no_linkage; }
 static int internal;
-namespace { extern int unique_external; }
+namespace { struct unique_external_type {} }
+unique_external_type unique_external;
 extern int external;
 """, lang = 'cpp')