[sanitizer] Allow BackgroundThread to not depend on StackDepot

Summary:
Still pursuing the ultimate goal of splitting the Symbolizer code from
RTSanitizerCommon core, allow `BackgroundThread` to work even when not linked
with `sanitizer_stackdepot.cc`. There is no reason this function should pull in
the whole `StackDepot` if symbolization is not supported.

Currently this has no functional change as the depot is always linked anyway.

Reviewers: alekseyshl

Reviewed By: alekseyshl

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

Differential Revision: https://reviews.llvm.org/D45296

llvm-svn: 329595