Simplify. NFC.

Two simplifications:
- We check `!Previous.empty()` above and only use `Previous` in const
  contexts after that check, so the `!Previous.empty()` check seems
  redundant.
- The null check looks pointless, as well: AFAICT, `LookupResults`
  should never contain null entries, and `OldDecl` should always be
  non-null if `Redeclaration` is true.

llvm-svn: 299601