Add an interesting testcase we already handle correctly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176890 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/SemaCXX/extern-c.cpp b/test/SemaCXX/extern-c.cpp
index c4b1141..c55b10d 100644
--- a/test/SemaCXX/extern-c.cpp
+++ b/test/SemaCXX/extern-c.cpp
@@ -44,3 +44,15 @@
 extern "C" {
   static float test5_b; // expected-error {{redefinition of 'test5_b' with a different type: 'float' vs 'int'}}
 }
+
+extern "C" {
+  void f() {
+    extern int test6_b;
+  }
+}
+namespace foo {
+  extern "C" {
+    static float test6_b;
+    extern float test6_b;
+  }
+}