Assert that redeclarations have the same linkage.

It is somewhat hard to test linkage, so I decided to try to add an assert. This
already found some interesting cases where there were different.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@171585 91177308-0d34-0410-b5e6-96231b3b80d8