[FuncUnwinders] Use "symbol file" unwind plans for unwinding

Summary:
Previous patch (r360409) introduced the "symbol file unwind plan"
concept, but that plan wasn't used for unwinding yet. With this patch,
we start to consider the new plan as a possible strategy for both
synchronous and asynchronous unwinding. I also add a test that asserts
that unwinding via breakpad STACK CFI info works end-to-end.

Reviewers: jasonmolenda, clayborg

Subscribers: lldb-commits, amccarth, markmentovai

Differential Revision: https://reviews.llvm.org/D61853

llvm-svn: 361618