Change the representation of dependent elaborated-type-specifiers

(such as "class T::foo") from an ElaboratedType of a TypenameType to a
DependentNameType, which more accurately models the underlying
concept.

Improve template instantiation for DependentNameType nodes that
represent nested-name-specifiers, by performing tag name lookup and
checking the resulting tag appropriately. Fixes PR5681.

There is still much testing and cleanup to do in this area.

llvm-svn: 100054