[modules] Rearrange and unify the way we determine if we need to pull in
redeclaration chains when pulling in a declaration. We need the redecl chain
unless we know some other declaration will trigger it to be pulled in; that
happens if our originally-canonical declaration had all the knowledge that
we have (and isn't us).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213043 91177308-0d34-0410-b5e6-96231b3b80d8