[NFC][Sanitizer] Add new BufferedStackTrace::Unwind API

[NFC][Sanitizer] Add new BufferedStackTrace::Unwind API

Add new Unwind API. This is the final envisioned API with the correct
abstraction level. It hides/slow fast unwinder selection from the caller
and doesn't take any arguments that would leak that abstraction (i.e.,
arguments like stack_top/stack_bottom).

GetStackTrace will become an implementation detail (private method) of
the BufferedStackTrace class.

Reviewers: vitalybuka

Differential Revision: https://reviews.llvm.org/D58741

llvm-svn: 355168