Narrow the -Wunsupported-dll-base-class-template warning.

Don't warn about not being able to propagate dll attribute to a base class template
when that base already has a different attribute.

MSVC doesn't actually try to do this; the first attribute that was propagated
takes precedence, so Clang is already doing the right thing and there's no
need to warn.

(This is a step towards fixing PR21718.)

llvm-svn: 239372