[clang][AST][NFC] Expose clang::isSubstitutedDefaultArgument to clang/AST consumers

**Summary**
A use-case has come up in DWARF CodeGen where we want to determine
whether a particular template argument matches the default template
parameter declaration.

Re-using this TypePrinter component there allows us to avoid duplicating
most of this code and immediately allows us to cover a wider range
of use-cases than the DWARF CodeGen does today.