[libclang 4/8] Add the clang_Type_getNullability() API

Summary:
This patch adds a clang-c API for querying the nullability of an AttributedType.

The test here also tests D49081

Reviewers: yvvan, jbcoe

Reviewed By: yvvan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49082

llvm-svn: 338809