[clang] NFCI: use TemplateArgumentLoc for NTTP DefaultArgument (#92852)

This is an enabler for https://github.com/llvm/llvm-project/pull/92855

This allows an NTTP default argument to be set as an arbitrary
TemplateArgument, not just an expression.
This allows template parameter packs to have default arguments in the
AST, even though the language proper doesn't support the syntax for it.

This allows NTTP default arguments to be other kinds of arguments, like
packs, integral constants, and such.