[sanitizer] Move the errno/ENOMEM allocator checks logic to separate .cc

Summary:
The fact that `sanitizer_allocator_checks.h` is including `sanitizer_errno.h`
creates complications for future changes, where it would conflict with `errno.h`
definitions on Android and Fuchsia (macro redefinition).

By moving the portion that sets errno in the checks to a separate compilation
unit, we avoid the inclusion of the header there, which solves the issue.

Not that it is not vital to have that function in a header as it is called as a
result of an unlikely event, and doesn't need to be inlined.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: kubamracek, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D38706

llvm-svn: 315319