Implement template instantiation for ClassTemplateSpecializationTypes,

such as replacing 'T' in vector<T>. There are a few aspects to this:

  - Extend TemplateArgument to allow arbitrary expressions (an
    Expr*), and switch ClassTemplateSpecializationType to store
    TemplateArguments rather than it's own type-or-expression
    representation.

  - ClassTemplateSpecializationType can now store dependent types. In
    that case, the canonical type is another
    ClassTemplateSpecializationType (with default template arguments
    expanded) rather than a declaration (we don't build Decls for
    dependent types).

  - Split ActOnClassTemplateId into ActOnClassTemplateId (called from
    the parser) and CheckClassTemplateId (called from
    ActOnClassTemplateId and InstantiateType). They're smart enough to
    handle dependent types, now.

llvm-svn: 66509