Introduce a new expression type, UnresolvedDeclRefExpr, that describes

Introduce a new expression type, UnresolvedDeclRefExpr, that describes
dependent qualified-ids such as

  Fibonacci<N - 1>::value

where N is a template parameter. These references are "unresolved"
because the name is dependent and, therefore, cannot be resolved to a
declaration node (as we would do for a DeclRefExpr or
QualifiedDeclRefExpr). UnresolvedDeclRefExprs instantiate to
DeclRefExprs, QualifiedDeclRefExprs, etc.

Also, be a bit more careful about keeping only a single set of
specializations for a class template, and instantiating from the
definition of that template rather than a previous declaration. In
general, we need a better solution for this for all TagDecls, because
it's too easy to accidentally look at a declaration that isn't the
definition.

We can now process a simple Fibonacci computation described as a
template metaprogram.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67308 91177308-0d34-0410-b5e6-96231b3b80d8