[clang][DebugInfo] Add DW_AT_default_value support for template template parameters

After this patch, in the following snippet:
```
template <typename T> Foo {};

template <template <typename T> class CT = Foo> Bar {};

Bar<> b;
```

The debug-info entry for the `CT` template parameter will have
a `DW_AT_default_value (true)` attached to it.

Differential Revision: https://reviews.llvm.org/D139989