[LSan] Make LSan allocator allocator_may_return_null compliant

Summary:
LSan allocator used to always return nullptr on too big allocation requests
(the definition of "too big" depends on platform and bitness), now it
follows policy configured by allocator_may_return_null flag.

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34786

llvm-svn: 306624