[clang][TypePrinter] Test TemplateArgument::IsDefaulted when omitting default arguments

**Summary**

This patch allows clients who can't properly construct
a `ClassTemplateDecl` to still benefit from the `clang::TypePrinter`s
ability to skip printing defaulted template arguments. The
clients simply have to call `TemplateArgument::setIsDefaulted`
in advance.

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

Differential Revision: https://reviews.llvm.org/D141827