A step towards sorting out handling of triviality of special members in C++11.

Separate out the notions of 'has a trivial special member' and 'has a
non-trivial special member', and use them appropriately. These are not
opposites of one another (there might be no special member, or in C++11 there
might be a trivial one and a non-trivial one). The CXXRecordDecl predicates
continue to produce incorrect results, but do so in fewer cases now, and
they document the cases where they might be wrong.

No functionality changes are intended here (they will come when the predicates
start producing the right answers...).

llvm-svn: 168119