[clang][ASTImporter] Propagate TemplateArgument::IsDefaulted during import

With https://reviews.llvm.org/D141826 `TemplateArgument`s have an
additional field that indicates their defaulted-ness. This gets
used during debug-info generation and in the `clang::TypePrinter`.

This patch copies the field during the import process so consumers
of the ASTImporter can benefit from the other Clang components that
read the field.

**Testing**

* Added unit-test
* Checked that this fixes (in addition to a follow-up LLDB patch)
  fix current test failures in LLDB

Differential Revision: https://reviews.llvm.org/D142713