Sema: Diagnose improper application of inheritance keywords

We would previously allow inappropriate inheritance keywords to appear
on class declarations.  We would also allow inheritance keywords on
templates which were not fully specialized; this was divergent from
MSVC.

Differential Revision: http://llvm-reviews.chandlerc.com/D2585


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200423 91177308-0d34-0410-b5e6-96231b3b80d8