[Sema] Compute the nullability of a conditional expression based on the

[Sema] Compute the nullability of a conditional expression based on the
nullabilities of its operands.

This patch defines a function to compute the nullability of conditional
expressions, which enables Sema to precisely detect implicit conversions
of nullable conditional expressions to nonnull pointers.

rdar://problem/25166556

Differential Revision: https://reviews.llvm.org/D22392

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276076 91177308-0d34-0410-b5e6-96231b3b80d8