Fix speculative parsing of dependent template names in

nested-name-specifiers so that they don't gobble the template name (or
operator-function-id) unless there is also a
template-argument-list. For example, given

  T::template apply

we would previously consume both "template" and "apply" as part of
parsing the nested-name-specifier, then error when we see that there
is no "<" starting a template argument list. Now, we parse such
constructs tentatively, and back off if the "<" is not present. This
allows us to parse dependent template names as one would use them for,
e.g., template template parameters:

  template<typename T, template<class> class X = T::template apply>
    struct MetaSomething;

Also, test default arguments for template template parameters.

llvm-svn: 86841