[Clang] Check for returns_nonnull when deciding to add allocation null checks

Non-throwing allocators currently will always get null-check code. However, if the non-throwing allocator is explicitly annotated with returns_nonnull the null check should be elided.

Testing:
ninja check-all
added test case correctly elides

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D102820