[ubsan] Relax nullability-return for blocks with deduced types

When the return type of an ObjC-style block literals is deduced, pick
the candidate type with the strictest nullability annotation applicable
to every other candidate.

This suppresses a UBSan false-positive in situations where a too-strict
nullability would be deduced, despite the fact that the returned value
would be implicitly cast to _Nullable.

rdar://41317163

llvm-svn: 335572