When name lookup for a redeclaration finds declarations that are known

When name lookup for a redeclaration finds declarations that are known
(because they are part of some module) but have not been made visible
(because they are in a submodule that wasn't imported), filter out
those declarations unless both the old declaration and the new
declaration have external linkage. When one or both has internal
linkage, there should be no conflict unless both are imported.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171925 91177308-0d34-0410-b5e6-96231b3b80d8