Don't instantiate members not belonging in the semantic context of the template.

Don't instantiate members not belonging in the semantic context of the template.
e.g. for:

    template <int i> class A {
      class B *g;
    };

'class B' has the template as lexical context but semantically it is
introduced in namespace scope.

Fixes rdar://8611125 & http://llvm.org/PR8505

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@118235 91177308-0d34-0410-b5e6-96231b3b80d8