Lay the ground work for resoving PR7047. This doesn't actually fix it because

default arguments to template parameters don't have a DeclContext when
instantiated, and so we can't detect that we're in an instantiation context as
opposed to the definition context. However, it fixes the more commonly-occuring
cases in TMP code that use devolve to this type of tautology after
substitution.

llvm-svn: 108044