[-fms-extensions] Don't crash on explicit class-scope specializations & default arguments

The code had a typo it was doing:
  Param->setUninstantiatedDefaultArg(Param->getUninstantiatedDefaultArg());

This is a no-op but may assert, we wanted to do:
  Param->setUninstantiatedDefaultArg(OldParam->getUninstantiatedDefaultArg());

This fixes PR28082.

llvm-svn: 272425