Give NullabilityKind a printing operator<<

This is more useful for debug/test than getNullabilitySpelling:
 - default form has uglifying underscores
 - non-default form crashes on NullableResult
 - both return unhelpfully verbose strings for Unspecified
 - operator<< works with gtest, formatv, etc

Differential Revision: https://reviews.llvm.org/D149650