Itanium ABI: Template template parameters are usable as substitutions

Template template parameters weren't added to the list of substitutions.
This would make the substitution map contain inaccurate mappings,
leading to Clang violating the Itanium ABI and breaking compatibility
with GCC.

This fixes PR21351.

Differential Revision: http://reviews.llvm.org/D5959
[modules] Support combining 'textual' with 'private'.