[Sanitizer] Introduce Allocator::may_return_null bool flag.

Summary:
Turn "allocator_may_return_null" common flag into an
Allocator::may_return_null bool flag. We want to make sure
that common flags are immutable after initialization. There
are cases when we want to change this flag in the allocator
at runtime: e.g. in unit tests and during ASan activation
on Android.

Test Plan: regression test suite, real-life applications

Reviewers: kcc, eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6623