[LSan] Introduce a callback mechanism to allow adding data reachable from...

[LSan] Introduce a callback mechanism to allow adding data reachable from ThreadContexts to the frontier.

This mechanism is intended to provide a way to treat the `arg` pointer
of a created (but not yet started) thread as reachable. In future
patches this will be implemented in `GetAdditionalThreadContextPtrs`.

A separate implementation of `GetAdditionalThreadContextPtrs` exists
for ASan and LSan runtimes because they need to be implemented
differently in future patches.

rdar://problem/63537240

Differential Revision: https://reviews.llvm.org/D95183