[clang] NFCI: use TemplateArgumentLoc for type-param DefaultArgument (#92854)

This is an enabler for a future patch.

This allows an type-parameter default argument to be set as an arbitrary
TemplateArgument, not just a type.
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 will be used in a later patch which synthesizes template parameter
lists with arbitrary default arguments taken from template
specializations.

There are a few places we used SubsType, because we only had a type, now
we use SubstTemplateArgument.
SubstTemplateArgument was missing arguments for setting Instantiation
location and entity names.
Adding those is needed so we don't regress in diagnostics.