[codeview] Don't emit variable templates as class members

MSVC doesn't, so neither should we.

Fixes PR38004, which is a crash that happens when we try to emit debug
info for a still-dependent partial variable template specialization.

As a follow-up, we should review what we're doing for function and class
member templates. It looks like we don't filter those out, but I can't
seem to get clang to emit any.

llvm-svn: 337616