We referred to all declaration in definitions in our diagnostic messages
which is can be inaccurate.  Instead, classify the declaration and emit
an appropriate diagnostic for the new declaration and an appropriate
note pointing to the old one.

This fixes PR24116.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@242190 91177308-0d34-0410-b5e6-96231b3b80d8