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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@108044 91177308-0d34-0410-b5e6-96231b3b80d8