Fix PR13784: instantiation of an abstract class in a conditional operator.

A couple of missing "RequireNonAbstractType" calls in conditional operator
handling. I looked for opportunities to tie this check in to all relevant
callers of PerformCopyInitialization (couldn't be all callers since this is
called for base subobject copying too, where it's acceptable to copy abstract
types) but the callers varied too much & in many cases had substantial code
or conditionals on the RequireNonAbstractType call, the
PerformCopyInitialization call, or the code between the two calls.

llvm-svn: 163555