Store "sugared" decls in LookupResults (i.e. decl aliases like using declarations);

strip the sugar off in getFoundDecl() and getAsSingleDecl(), but leave it on for
clients like overload resolution who want to use the iterators.

Refactor a few pieces of overload resolution to strip off using declarations in
a single place.  Don't do anything useful with the extra context knowledge yet.

llvm-svn: 89061