Track definition merging on the canonical declaration even when local

Track definition merging on the canonical declaration even when local
submodule visibility is disabled.

Attempting to pick a specific declaration to make visible when the
module containing the merged declaration becomes visible is error-prone,
as we don't yet know which declaration we'll choose to be the definition
when we are informed of the merging.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@342019 91177308-0d34-0410-b5e6-96231b3b80d8