[sanitizer_common] Create max_allocation_size_mb flag.

Summary:
The flag allows the user to specify a maximum allocation size that the
sanitizers will honor.  Any larger allocations will return nullptr or
crash depending on allocator_may_return_null.

Reviewers: kcc, eugenis

Reviewed By: kcc, eugenis

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69576