we have a redeclarable type, and only use the new virtual versions
(getPreviousDeclImpl() and getMostRecentDeclImpl()) when we don't have
that type information. This keeps us from penalizing users with strict
type information (and is the moral equivalent of a "final" method).

Plus, settle on the names getPreviousDecl() and getMostRecentDecl()
throughout.

llvm-svn: 148187