[modules] Prefer more complete array types.

[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.

llvm-svn: 262189