[TTC-8][Sanitizers] 32 bit allocator respects allocator_may_return_null flag

[TTC-8][Sanitizers] 32 bit allocator respects allocator_may_return_null flag

Summary:
Make SizeClassAllocator32 return nullptr when it encounters OOM, which
allows the entire sanitizer's allocator to follow allocator_may_return_null=1
policy, even for small allocations (LargeMmapAllocator is already fixed
by D34243).

Will add a test for OOM in primary allocator later, when
SizeClassAllocator64 can gracefully handle OOM too.

Reviewers: eugenis

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D34433

Change-Id: I2853a00cff5202189c5d4b15b1934c63d5206708
git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@305972 91177308-0d34-0410-b5e6-96231b3b80d8