Fix PR32097 - is_abstract doesn't work on class templates.

This patch fixes llvm.org/PR32097 by using the __is_abstract
builtin type-trait instead of the previous library-only implementation.

All supported compilers provide this trait. I've tested as far
back as Clang 3.2, GCC 4.6 and MSVC trunk.

llvm-svn: 296561