Sign-extend addresses in CompactRingBuffer.

Summary:
This is neccessary to support solaris/sparc9 where some userspace
addresses have all top bits set, as well as, potentially, kernel memory
on aarch64.

This change does not update the compiler side (HWASan IR pass) which
needs to be done separately for the affected targets.

Reviewers: ro, vitalybuka

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D91827