[lldb] Add support for DW_AT_default_value in template params

**Summary**

This patch makes LLDB understand the `DW_AT_default_value` on
template argument DIEs. As a result, type summaries will no
longer contain the defaulted template arguments, reducing
noise substantially. E.g.,

Before:
```
(lldb) v nested
(std::vector<std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::allocator<std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator <char> > > > > >) nested = size=0 {}
```

After:
```
(lldb) v nested
(std::vector<std::vector<std::basic_string<char> > >) nested = size=0 {}
```

See discussion in https://reviews.llvm.org/D140423

**Testing**

* Adjust API tests
* Added unit-test

Differential Revision: https://reviews.llvm.org/D141828