[Sanitizers] Secondary allocator respects allocator_may_return_null=1.

Summary:
Context: https://github.com/google/sanitizers/issues/740.

Making secondary allocator to respect allocator_may_return_null=1 flag
and return nullptr when "out of memory" happens.

More changes in primary allocator and operator new will follow.

Reviewers: eugenis

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D34243

llvm-svn: 305569