Incremental progress on using declarations. Split UnresolvedUsingDecl into

two classes, one for typenames and one for values;  this seems to have some
support from Doug if not necessarily from the extremely-vague-on-this-point
standard.  Track the location of the 'typename' keyword in a using-typename
decl.  Make a new lookup result for unresolved values and deal with it in
most places.

llvm-svn: 89184