[LSan] Make LSan allocator allocator_may_return_null compliant

Summary:
An attempt to reland D34786 (which caused bot failres on Mac), now with
properly intercepted operators new() and delete().

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/D34845

llvm-svn: 306845