Push nested-name-specifier source-location information into dependent

template specialization types. This also required some parser tweaks,
since we were losing track of the nested-name-specifier's source
location information in several places in the parser. Other notable
changes this required:

  - Sema::ActOnTagTemplateIdType now type-checks and forms the
    appropriate type nodes (+ source-location information) for an
    elaborated-type-specifier ending in a template-id. Previously, we
    used a combination of ActOnTemplateIdType and
    ActOnTagTemplateIdType that resulted in an ElaboratedType wrapped
    around a DependentTemplateSpecializationType, which duplicated the
    keyword ("class", "struct", etc.) and nested-name-specifier
    storage.

  - Sema::ActOnTemplateIdType now gets a nested-name-specifier, which
    it places into the returned type-source location information.

  - Sema::ActOnDependentTag now creates types with source-location
    information.

llvm-svn: 126808