[sanitizer] Allow BackgroundThread to not depend on StackDepot v2

Summary:
This is a redo of D45296.
It looks like the random stack-protector issues I was getting were coming from
my Android emulator, and updating everything all around and relaunching stuff
ended up making it go away.
I guess I'll have to see how it behaves on the bots.
Only additional change from the previous CL is some `const` were appropriate.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D45461

llvm-svn: 329706