Turn off __has_feature(cxx_rtti) when -fno-rtti-data is present

-fno-rtti-data makes it so that vtables emitted in the current TU lack
RTTI data. This means that dynamic_cast usually fails at runtime. Users
of the existing cxx_rtti feature expect all of RTTI to work, not just
some of it.

Chromium bug for context: http://crbug.com/518191

llvm-svn: 244922