[modules] Prefer more complete array types.

If we import a module that has a complete array type and one that has an
incomplete array type, the declaration found by name lookup might be the one with
the incomplete type, possibly resulting in rejects-valid.

Now, the name lookup prefers decls with a complete array types. Also,
diagnose cases when the redecl chain has array bound, different from the merge
candidate.

Reviewed by Richard Smith.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262189 91177308-0d34-0410-b5e6-96231b3b80d8