Disallow extern decls of type void in C++ mode

C++ and C differ with respect to the handling of extern void
declarations. Enforce the C++ behavior in C++ mode.

llvm-svn: 182814
diff --git a/clang/test/SemaCXX/linkage-spec.cpp b/clang/test/SemaCXX/linkage-spec.cpp
index 504df0d..7681c7d 100644
--- a/clang/test/SemaCXX/linkage-spec.cpp
+++ b/clang/test/SemaCXX/linkage-spec.cpp
@@ -114,3 +114,5 @@
   }
   int js::ObjectClass;
 }
+
+extern "C" void PR16167; // expected-error {{variable has incomplete type 'void'}}