[Sanitizers] 64 bit allocator respects allocator_may_return_null flag

Summary:
Make SizeClassAllocator64 return nullptr when it encounters OOM, which
allows the entire sanitizer's allocator to follow
allocator_may_return_null=1 policy
(LargeMmapAllocator: D34243, SizeClassAllocator64: D34433).

Reviewers: eugenis

Subscribers: srhines, kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D34540

llvm-svn: 306342