Ignore -Wunsupported-dll-base-class-template by default

The situation it is warning about (see PR20725) is not very likely
to be a real problem, and it is unclear what action the user should take
if the warning does fire.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216283 91177308-0d34-0410-b5e6-96231b3b80d8