Correctly compute linkage of decls forward declared extern C.

This fixes a crash in

namespace {
  struct X {};
}
extern "C" X test2_b;
X test2_b

before we would assign different linkages to each of the test2_b decls.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176869 91177308-0d34-0410-b5e6-96231b3b80d8