[clang][TypePrinter] Support expression template arguments when checking defaultedness

This patch adds support for `TemplateArgument`s of kind
`TemplateArgument::Expression` to `clang::isSubstitutedDefaultArgument`.
We do so by evaluating both the `Pattern` and `Arg` expression to an
`APInt`, if we can, and comparing the results.

This will be useful in an upcoming change where
`clang::isSubstitutedDefaultArgument` gets called from `clang::Sema`
where the `TemplateArgument`s are instantiated as expressions (without
being evaluted to `APInt` beforehand).

**Testing**

- Added unit-tests

Differential Revision: https://reviews.llvm.org/D142632