[lsan] Add an interface function for on-demand leak checking.

Summary:
Add an interface function which can be used to periodically trigger
leak detection in a long-running process.

NB: The meaning of the kIgnored tag has been changed to allow easy clean-up
between subsequent leak checks. Previously, this tag was applied to explicitly
ignored (i.e. with __lsan_disable() or __lsan_ignore_object()) chunks *and* any
chunks only reachable from those. With this change, it's only applied to
explicitly ignored chunks.

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9159

llvm-svn: 235728