Propagate the "found declaration" (i.e. the using declaration instead of

the underlying/instantiated decl) through a lot of API, including "intermediate"
MemberExprs required for (e.g.) template instantiation.  This is necessary
because of the access semantics of member accesses to using declarations:
only the base class *containing the using decl* need be accessible from the
naming class.

This allows us to complete an access-controlled selfhost, if there are no
recent regressions.

llvm-svn: 99936