[modules] If we find more formerly-canonical declarations of an entity while

building its redecl chains, make sure we pull in the redeclarations of those
canonical declarations.

It's pretty difficult to reach a situation where we can find more canonical
declarations of an entity while building its redecl chains; I think the
provided testcase (4 modules and 7 declarations) cannot be reduced further.

llvm-svn: 232411