[clang] Make sure attributes on member classes are applied properly

Summary:
Attributes on member classes of class templates and member class templates
of class templates are not currently instantiated. This was discovered by
Richard Smith here:

  http://lists.llvm.org/pipermail/cfe-dev/2018-September/059291.html

This commit makes sure that attributes are instantiated properly. This
commit does not fix the broken behavior for member partial and explicit
specializations of class templates.

PR38913

Reviewers: rsmith

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D51997

llvm-svn: 342238