Implement declaration merging for typedefs loaded from disjoint

Implement declaration merging for typedefs loaded from disjoint
modules, so long as the typedefs refer to the same underlying
type. This ensures that the typedefs end up in the same redeclaration
chain.

To test this, fix name lookup for C/Objective-C to properly deal with
multiple declarations with the same name in the same scope.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147533 91177308-0d34-0410-b5e6-96231b3b80d8