Re-land r250592 without rejecting field refs in unevaluated contexts

This time, I went with the first approach from
http://reviews.llvm.org/D6700, where clang actually attempts to form an
implicit member reference from an UnresolvedLookupExpr. We know that
there are only two possible outcomes at this point, a DeclRefExpr of the
FieldDecl or an error, but its safer to reuse the existing machinery for
this.

llvm-svn: 250856