[lldb][CXXModuleHandler] Set TemplateArgument::IsDefaulted

Since https://reviews.llvm.org/D141827 this field gets used
to print type names. Thus propagate it after importing TemplateArguments
from modules.

**Testing**

* Confirmed that this fixes the ongoing `import-std-module` test
  failures.

Differential Revision: https://reviews.llvm.org/D142714