[TTC-8][Sanitizers] 64 bit allocator respects allocator_may_return_null flag

[TTC-8][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

Change-Id: I0e72b4b22a440eb644dcc370da0ba9aa0e71c2d5
git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@306342 91177308-0d34-0410-b5e6-96231b3b80d8