[Sanitizer] On Darwin `__sanitizer_print_stack_trace` only prints topmost frame

In compiler-rt we have the notion of a `fast` and a `slow` stack
unwinder. Darwin currently only supports the fast unwinder.

From reading the code, my understanding is that
`BufferedStackTrace::Unwind` can be called with `bp=0, stack_top=0,
stack_bottom=0, request_fast_unwind=false`. If
`request_fast_unwind=true`, then we alos need to supply bp, stack_top,
and stack_bottom.

However, `BufferedStackTrace::Unwind` uses
`StackTrace::WillUseFastUnwind` which will adapt `request_fast_unwind`
if the requested unwinder is not supported. On Darwin, the result is
that we don't pass actual values for bp, stack_top, and stack_bottom,
but end up using the fast unwinder. The tests then fail because we only
print the topmost stack frame.

This patch adds a check to `WillUseFastUnwind` at the point of usage to
avoid the mismatch between `request_fast_unwind` and what `Unwind`
actually does. I am also interested in cleaning up the
`request_fast_unwind` machinery so this patch just the simplest thing
possible so I can enable the tests.

Reviewers: vitalybuka, vsk

Differential Revision: https://reviews.llvm.org/D58156

llvm-svn: 354282